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Complement system
Complement system
Complement system
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Scheme of the complement system

The complement system, also known as complement cascade, is a part of the humoral, innate immune system and enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and attack the pathogen's cell membrane.[1] Despite being part of the innate immune system, the complement system can be recruited and brought into action by antibodies generated by the adaptive immune system.

The complement system consists of a number of small, inactive, liver synthesized protein precursors circulating in the blood. When stimulated by one of several triggers, proteases in the system cleave specific proteins to release cytokines and initiate an amplifying cascade of further cleavages. The end result of this complement activation or complement fixation cascade is stimulation of phagocytes to clear foreign and damaged material, inflammation to attract additional phagocytes, and activation of the cell-killing membrane attack complex. About 50 proteins and protein fragments make up the complement system, including plasma proteins, and cell membrane receptors. They account for about 10% of the globulin fraction of blood serum.[2]

Three biochemical pathways activate the complement system: the classical complement pathway, the alternative complement pathway, and the lectin pathway.[3] The alternative pathway accounts for the majority of terminal pathway activation and so therapeutic efforts in disease have revolved around its inhibition.[4]

History

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In 1888, George Nuttall found that sheep blood serum had mild killing activity against the bacterium that causes anthrax.[5] The killing activity disappeared when he heated the blood.[6] In 1891, Hans Ernst August Buchner, noting the same property of blood in his experiments, named the killing property "alexin", which means "to ward off" in Greek.[7][8] By 1894, several laboratories had demonstrated that serum from guinea pigs that had recovered from cholera killed the cholera bacterium in vitro. Heating the serum destroyed its killing activity. Nevertheless, the heat-inactivated serum, when injected into guinea pigs exposed to the cholera bacteria, maintained its ability to protect the animals from illness. Jules Bordet, a young Belgian scientist in Paris at the Pasteur Institute, concluded that this principle has two components, one that maintained a "sensitizing" effect after being heated and one (alexin) whose toxic effect was lost after being heated.[9] The heat-stable component was responsible for immunity against specific microorganisms, whereas the heat-sensitive component was responsible for the non-specific antimicrobial activity conferred by all normal sera. In 1899, Paul Ehrlich renamed the heat-sensitive component "complement."[10][6]

Ehrlich introduced the term "complement" as part of his larger theory of the immune system.[11] According to this theory, the immune system consists of cells that have specific receptors on their surfaces to recognize antigens. Upon immunization with an antigen, more of these receptors are formed, and they are then shed from the cells to circulate in the blood. Those receptors, which we now call "antibodies," were called by Ehrlich "amboceptors" to emphasise their bifunctional binding capacity: They recognise and bind to a specific antigen, but they also recognise and bind to the heat-labile antimicrobial component of fresh serum. Ehrlich, therefore, named this heat-labile component "complement," because it is something in the blood that "complements" the cells of the immune system. Ehrlich believed that each antigen-specific amboceptor has its own specific complement, whereas Bordet believed that there is only one type of complement. In the early 20th century, this controversy was resolved when it became understood that complement can act in combination with specific antibodies, or on its own in a non-specific way.[citation needed]

Functions

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Membrane attack complex (Terminal Complement Complex C5b-9)

Complement triggers the following immune functions:[12]

  1. Membrane attack – by rupturing the cell wall of bacteria. (classical complement pathway)
  2. Phagocytosis – by opsonizing antigens. C3b has most important opsonizing activity. (alternative complement pathway)
  3. Inflammation – by attracting macrophages and neutrophils. (lectin pathway)

Overview

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Most of the proteins and glycoproteins that constitute the complement system are synthesized by hepatocytes. But significant amounts are also produced by tissue macrophages, blood monocytes, and epithelial cells of the genitourinary system and gastrointestinal tract. The three pathways of activation all generate homologous variants of the protease C3-convertase. The classical complement pathway typically requires antigen-antibody complexes for activation (specific immune response), whereas the alternative pathway can be activated by spontaneous complement component 3 (C3) hydrolysis, foreign material, pathogens, or damaged cells. The mannose-binding lectin pathway can be activated by C3 hydrolysis or antigens without the presence of antibodies (non-specific immune response). In all three pathways, C3-convertase cleaves and activates component C3, creating C3a and C3b, and causes a cascade of further cleavage and activation events. C3b binds to the surface of pathogens, leading to greater internalization by phagocytic cells by opsonization.[citation needed]

In the alternative pathway, C3b binds to Factor B. Factor D releases Factor Ba from Factor B bound to C3b. The complex of C3b(2)Bb is a protease which cleaves C5 into C5b and C5a. C5 convertase is also formed by the classical pathway when C3b binds C4b and C2b. C5a is an important chemotactic protein, helping recruit inflammatory cells. C3a is the precursor of an important cytokine (adipokine) named ASP (although this is not universally accepted [13]) and is usually rapidly cleaved by carboxypeptidase B. Both C3a and C5a have anaphylatoxin activity, directly triggering degranulation of mast cells as well as increasing vascular permeability and smooth muscle contraction.[13] C5b initiates the membrane attack pathway, which results in the membrane attack complex (MAC), consisting of C5b, C6, C7, C8, and polymeric C9.[14] MAC is the cytolytic endproduct of the complement cascade; it forms a transmembrane channel, which causes osmotic lysis of the target cell. Kupffer cells and other macrophage cell types help clear complement-coated pathogens. As part of the innate immune system, elements of the complement cascade can be found in species earlier than vertebrates; most recently in the protostome horseshoe crab species, putting the origins of the system back further than was previously thought.[citation needed]

Reaction cascade of the complement system: classical, alternative, and lectin pathways, amplification loop, terminal pathway, and membrane attack complex.

Classical pathway

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Main article: Classical complement pathway
The classical and alternative complement pathways

The classical pathway is triggered by activation of the C1-complex. The C1-complex is composed of 1 molecule of C1q, 2 molecules of C1r and 2 molecules of C1s, or C1qr2s2. This occurs when C1q binds to IgM or IgG complexed with antigens. A single pentameric IgM can initiate the pathway, while several, ideally six, IgGs are needed. This also occurs when C1q binds directly to the surface of the pathogen. Such binding leads to conformational changes in the C1q molecule, which leads to the activation of two C1r molecules. C1r is a serine protease. They then cleave C1s (another serine protease). The C1r2s2 component now splits C4 and then C2, producing C4a, C4b, C2a, and C2b (historically, the larger fragment of C2 was called C2a but is now referred to as C2b). C4b and C2b bind to form the classical pathway C3-convertase (C4b2b complex), which promotes cleavage of C3 into C3a and C3b. C3b later joins with C4b2b to make C5 convertase (C4b2b3b complex).[15]

Alternative pathway

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Main article: Alternative complement pathway

The alternative pathway is continuously activated at a low level, analogous to a car engine at idle, as a result of spontaneous C3 hydrolysis due to the breakdown of the internal thioester bond (C3 is mildly unstable in aqueous environment). The alternative pathway does not rely on pathogen-binding antibodies like the other pathways.[3] C3b that is generated from C3 by a C3 convertase enzyme complex in the fluid phase is rapidly inactivated by factor H and factor I, as is the C3b-like C3 that is the product of spontaneous cleavage of the internal thioester. In contrast, when the internal thioester of C3 reacts with a hydroxyl or amino group of a molecule on the surface of a cell or pathogen, the C3b that is now covalently bound to the surface is protected from factor H-mediated inactivation. The surface-bound C3b may now bind factor B to form C3bB. This complex in the presence of factor D will be cleaved into Ba and Bb. Bb will remain associated with C3b to form C3bBb, which is the alternative pathway C3 convertase.[16]

The C3bBb complex is stabilized by binding oligomers of factor P (properdin). The stabilized C3 convertase, C3bBbP, then acts enzymatically to cleave much more C3, some of which becomes covalently attached to the same surface as C3b. This newly bound C3b recruits more B, D and P activity and greatly amplifies the complement activation. When complement is activated on a cell surface, the activation is limited by endogenous complement regulatory proteins, which include CD35, CD46, CD55 and CD59, depending on the cell. Pathogens, in general, don't have complement regulatory proteins (there are many exceptions, which reflect adaptation of microbial pathogens to vertebrate immune defenses). Thus, the alternative complement pathway is able to distinguish self from non-self on the basis of the surface expression of complement regulatory proteins. Host cells don't accumulate cell surface C3b (and the proteolytic fragment of C3b called iC3b) because this is prevented by the complement regulatory proteins, while foreign cells, pathogens and abnormal surfaces may be heavily decorated with C3b and iC3b. Accordingly, the alternative complement pathway is one element of innate immunity.[citation needed]

Once the alternative C3 convertase enzyme is formed on a pathogen or cell surface, it may bind covalently another C3b, to form C3bBbC3bP, the C5 convertase. This enzyme then cleaves C5 to C5a, a potent anaphylatoxin, and C5b. The C5b then recruits and assembles C6, C7, C8 and multiple C9 molecules to assemble the membrane attack complex. This creates a hole or pore in the membrane that can kill or damage the pathogen or cell.[1]

Lectin pathway

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Main article: Lectin pathway

The lectin pathway is homologous to the classical pathway, but with the opsonin, mannose-binding lectin (MBL), and ficolins, instead of C1q. This pathway is activated by binding of MBL to mannose residues on the pathogen surface, which activates the MBL-associated serine proteases, MASP-1, and MASP-2 (very similar to C1r and C1s, respectively), which can then split C4 into C4a and C4b and C2 into C2a and C2b. C4b and C2b then bind together to form the classical C3-convertase, as in the classical pathway. Ficolins are homologous to MBL and function via MASP in a similar way. Several single-nucleotide polymorphisms have been described in M-ficolin in humans, with effect on ligand-binding ability and serum levels. Historically, the larger fragment of C2 was named C2a, but it is now referred to as C2b.[17] In invertebrates without an adaptive immune system, ficolins are expanded and their binding specificities diversified to compensate for the lack of pathogen-specific recognition molecules.[citation needed]

Complement protein fragment nomenclature

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Immunology textbooks have used different naming assignments for the smaller and larger fragments of C2 as C2a and C2b. The preferred assignment appears to be that the smaller fragment be designated as C2a: as early as 1994, a well known textbook recommended that the larger fragment of C2 should be designated C2b.[18] However, this was amplified in their 1999 4th edition, to say that:[19] "It is also useful to be aware that the larger active fragment of C2 was originally designated C2a, and is still called that in some texts and research papers. Here, for consistency, we shall call all large fragments of complement b, so the larger fragment of C2 will be designated C2b. In the classical and lectin pathways the C3 convertase enzyme is formed from membrane-bound C4b with C2b."[19]

This nomenclature is used in another literature:[20] The assignment is mixed in the latter literature, though. Some sources designate the larger and smaller fragments as C2a and C2b respectively[21][22][23][24][25][26][27][28][29] while other sources apply the converse.[18][19][30][31][32] However, due to the widely established convention, C2b here is the larger fragment, which, in the classical pathway, forms C4b2b (classically C4b2a). It may be noteworthy that, in a series of editions of Janeway's book, 1st to 7th, in the latest edition[28] they withdraw the stance to indicate the larger fragment of C2 as C2b.

Viral inhibition

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Fixation of the MBL protein on viral surfaces has also been shown to enhance neutralization of viral pathogens.[33]

Review

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Activation pathway Classic Alternative Lectin
Activator Ag–Ab Complex spontaneous hydrolysis of C3 MBL-Mannose Complex
C3-convertase C4b2b C3bBb C4b2b
C5-convertase C4b2b3b C3bBbC3b C4b2b3b
MAC development C5b+C6+C7+C8+C9

Activation of complements by antigen-associated antibody

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In the classical pathway, C1 binds with its C1q subunits to Fc fragments (made of CH2 region) of IgG or IgM, which has formed a complex with antigens. C4b and C3b are also able to bind to antigen-associated IgG or IgM, to its Fc portion.[20][25][28]

Such immunoglobulin-mediated binding of the complement may be interpreted as that the complement uses the ability of the immunoglobulin to detect and bind to non-self antigens as its guiding stick. The complement itself can bind non-self pathogens after detecting their pathogen-associated molecular patterns (PAMPs),[28] however, utilizing specificity of the antibody, complements can detect non-self targets much more specifically.[citation needed]

Some components have a variety of binding sites. In the classical pathway, C4 binds to Ig-associated C1q and C1r2s2 enzyme cleaves C4 to C4b and 4a. C4b binds to C1q, antigen-associated Ig (specifically to its Fc portion), and even to the microbe surface. C3b binds to antigen-associated Ig and to the microbe surface. Ability of C3b to bind to antigen-associated Ig would work effectively against antigen-antibody complexes to make them soluble.[citation needed]

Regulation

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The complement system has the potential to be extremely damaging to host tissues, meaning its activation must be tightly regulated. The complement system is regulated by complement control proteins, which are present at blood plasma and host cell membrane.[34] Some complement control proteins are present on the membranes of self-cells preventing them from being targeted by complement. One example is CD59, also known as protectin, which inhibits C9 polymerization during the formation of the membrane attack complex. The classical pathway is inhibited by C1-inhibitor, which binds to C1 to prevent its activation.[35] Another example, is a plasma protein called, Factor H (FH), which has a key role in down-regulating the alternative pathway.[36] Factor H, along with another protein called Factor I, inactivates C3b, the active form of C3. This process prevents the formation of C3 convertase and halts the progression of the complement cascade. C3-convertase also can be inhibited by decay accelerating factor (DAF), which is bound to erythrocyte plasma membranes via a GPI anchor.[35]

Role in disease

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Complement deficiency

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Main article: Complement deficiency

It is thought that the complement system might play a role in many diseases with an immune component, such as Barraquer–Simons syndrome, asthma, lupus erythematosus, glomerulonephritis, various forms of arthritis, autoimmune heart disease, multiple sclerosis, inflammatory bowel disease, paroxysmal nocturnal hemoglobinuria, atypical hemolytic uremic syndrome and ischemia-reperfusion injuries,[37][38] and rejection of transplanted organs.[39]

Complement regulation is suggested to play a role in pregnancy. Improper alternative complement pathway activation may mediate recurrent immune-mediated fetal loss.[40][41]

The complement system is also becoming increasingly implicated in diseases of the central nervous system such as Alzheimer's disease and other neurodegenerative conditions such as spinal cord injuries.[42][43][44]

Deficiencies of the terminal pathway predispose to both autoimmune disease and infections (particularly Neisseria meningitidis, due to the role that the membrane attack complex ("MAC") plays in attacking Gram-negative bacteria).[45]

Infections with N. meningitidis and N. gonorrhoeae are the only conditions known to be associated with deficiencies in the MAC components of complement.[46] 40–50% of those with MAC deficiencies experience recurrent infections with N. meningitidis.[47]

Deficiencies in complement regulators

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Mutations in the genes of complement regulators, especially factor H, have been associated with atypical hemolytic uremic syndrome,[4][48][49] and C3 glomerulopathy.[4] Both of these disorders are currently thought to be due to complement overactivation either on the surface of host cells or in plasma, with the molecular location of genetic variation in complement proteins providing clues into the underlying disease processes.[4] Moreover, several single nucleotide polymorphisms and mutations in the complement factor H gene (the most common of which results in the protein change p.Y402H) have been associated with the common eye disease age-related macular degeneration.[4] Polymorphisms of complement component 3, complement factor B, and complement factor I, as well as deletion of complement factor H-related 3 and complement factor H-related 1, also affect a person's risk of developing age-related macular degeneration.[4][50]

Mutations in the C1 inhibitor gene can cause hereditary angioedema, a genetic condition resulting from reduced regulation of bradykinin by C1-INH.[citation needed]

Paroxysmal nocturnal hemoglobinuria is caused by complement breakdown of RBCs due to an inability to make GPI. Thus the RBCs are not protected by GPI anchored proteins such as DAF.[51]

Diagnostic tools

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Diagnostic tools to measure complement activity include the total complement activity test.[52]

The presence or absence of complement fixation upon a challenge can indicate whether particular antigens or antibodies are present in the blood. This is the principle of the complement fixation test.[citation needed]

Modulation of the body by complement with infection

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Excessive complement activity contributes to severe Covid-19 symptoms and disease.[53] Although complement is intended to protect the body systems, under stress there can be more damage than protection. Research has suggested that the complement system is manipulated during HIV/AIDS, in a way that further damages the body.[54]

Role in the brain

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Research from over the last decade has shown that complement proteins of the classical complement pathway have an important role in synaptic pruning in the brain during early development.[55][56]

References

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[edit]
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Complement system

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The complement system is a critical component of the , comprising a network of more than 40 soluble and membrane-bound proteins that work together to detect and eliminate pathogens, damaged cells, and immune complexes, while also bridging innate and adaptive immunity. It enhances antibody-mediated responses and promotes , , and direct cell through tightly regulated cascades. The system includes central components such as the recognition molecules C1q and mannose-binding lectin (MBL), along with alternative pathway components like factor B, and effector proteins like C3 (the central molecule) and the terminal components C5–C9 that form the membrane attack complex (MAC). Complement activation occurs via three primary biochemical pathways: the classical pathway, initiated by antigen-antibody complexes binding C1q or direct C1q recognition of surfaces; the , triggered by MBL or ficolins binding carbohydrate patterns on microbes; and the alternative pathway, spontaneously activated by low-level C3 hydrolysis and amplified on foreign surfaces lacking host regulators. These pathways converge at the cleavage of C3 into C3a and C3b, leading to downstream amplification and formation of , which initiates the terminal pathway. Key functions of the complement system encompass opsonization of pathogens and apoptotic cells via C3b deposition to facilitate , generation of anaphylatoxins C3a and C5a to recruit immune cells and induce , and assembly of the MAC to lyse target cells by creating pores in their membranes. It also plays roles in immune , clearance of immune complexes, B-cell , and T-cell , thereby integrating innate defenses with adaptive responses. Dysregulation can lead to excessive or , contributing to diseases like systemic lupus erythematosus, , and age-related macular degeneration. To prevent host tissue damage, the complement system is tightly controlled by soluble regulators like and , and membrane-bound proteins such as (DAF/CD55) and membrane cofactor protein (MCP/CD46), which inhibit convertase activity and promote degradation of activated components. This regulatory balance ensures effective elimination while maintaining , with approved therapeutics such as complement inhibitors and ongoing research in modulating the system for autoimmune, infectious, and inflammatory disorders.

Introduction

Definition and components

The complement system is a crucial part of the , consisting of more than 30 soluble proteins in the plasma and membrane-bound proteins on host cells that function through a tightly regulated proteolytic cascade to detect and respond to pathogens. This network of proteins, amounting to about 3-5% of total plasma protein, enables rapid host defense by recognizing microbial surfaces and initiating effector mechanisms. Complement components are broadly classified by their primary roles in the cascade. Recognition molecules initiate by binding to pathogen-associated patterns; examples include C1q, which recognizes antibody-opsonized surfaces, and mannose-binding lectin (MBL), which binds to motifs on microbes. proteases propagate the cascade through sequential cleavage; key examples are C1r and C1s, which form part of the C1 complex, and factor B, which participates in alternative pathway convertase formation. Anaphylatoxins are inflammatory mediators released upon cleavage of central components, such as C3a and C5a, which recruit immune cells and induce . Opsonins tag targets for , including C3b, which covalently attaches to surfaces, and its inactivated form iC3b. Finally, membrane attack complex (MAC) proteins assemble into a pore-forming structure; these comprise C5b joined by C6, C7, C8, and multiple C9 molecules to form C5b-9. The proteins of the complement system fall into three major families based on structural and functional homology. Serine proteases, such as C1r, C1s, factor B, and factor D, contain catalytic domains that drive proteolytic steps. The C3/C4/C5 family consists of large, multi-domain proteins with an internal bond that enables covalent attachment to targets upon ; C3 serves as the central hub, while C4 and C5 contribute to amplification and terminal effects. Regulators of complement (RCA), also known as complement control proteins, inhibit uncontrolled activity to protect host tissues; this family includes soluble factors like and membrane-bound proteins such as CR1 (CD35) and (DAF, CD55).

Overview of functions

The complement system plays a central role in innate immunity by directly pathogens through the formation of the membrane attack complex (MAC), a transmembrane pore composed of C5b-9 proteins that causes osmotic of target cells such as . It also mediates opsonization, primarily via the deposition of C3b fragments on surfaces, which marks them for enhanced recognition and engulfment by expressing complement receptors. Additionally, activation generates anaphylatoxins C3a and C5a, which bind to receptors on mast cells, , and other immune cells to promote , , and recruitment of neutrophils and monocytes to infection sites, thereby amplifying local inflammatory responses. Complement further contributes to the solubilization and clearance of immune complexes, preventing their harmful deposition in tissues and facilitating their removal by the . Beyond direct antimicrobial effects, the complement system acts as an amplifier that bridges innate and adaptive immunity, enhancing the efficiency of antibody-mediated responses through interactions with B cells and antigen-presenting cells. For instance, complement-opsonized antigens bind to complement receptors on B cells (such as CR2/CD21), lowering the threshold for B-cell activation and promoting formation, which leads to improved antibody affinity maturation and class switching. In homeostatic contexts, complement maintains tissue integrity by facilitating the non-inflammatory clearance of apoptotic cells, where C1q and iC3b opsonize dying cells for efficient without triggering excessive . It also supports B-cell tolerance by regulating self-reactive B cells; for example, iC3b binding to complement receptors on immature B cells induces anergy or , preventing .

History

Early discoveries

The complement system's discovery began in the late 19th century with observations of antibacterial activity in blood serum. In 1891, German bacteriologist Hans Buchner identified a heat-labile factor in normal serum that could lyse , naming it "alexin" (from the Greek word meaning "to ward off"). This marked the initial recognition of a non-specific defensive component in blood, distinct from cellular immunity. Building on this, Belgian immunologist , working at the in 1894–1895, conducted pivotal experiments showing that immune serum's lytic effects on or foreign red blood cells required two distinct components: a heat-stable, specific "sensitizer" (later termed ) and a heat-labile, non-specific factor akin to Buchner's alexin. Bordet demonstrated that the heat-labile factor was universally present in fresh normal serum and could restore lytic activity when added to heated immune serum. In 1900, Bordet advanced this understanding through experiments on the complement-fixation reaction, revealing that antigen-antibody complexes could bind and "fix" the heat-labile factor, preventing it from participating in subsequent lysis of indicator cells. This observation distinguished the specific immune recognition from the amplifying role of complement and formed the basis for serological diagnostics. For these foundational contributions to immunity, including the elucidation of complement's role, Bordet received the 1919 Nobel Prize in Physiology or Medicine. Concurrent with Bordet's work, German scientist Paul Ehrlich, in the early 1900s, integrated complement into his side-chain theory of immunity, positing that antibodies (termed amboceptors) specifically fixed complement to target and destroy pathogens or altered cells. Ehrlich coined the term "complement" in 1899 to denote this serum factor that "complemented" antibody action in immune reactions, including fixation phenomena. His theoretical framework emphasized complement's enzymatic-like role in cytolysis, influencing early immunological research.

Key developments and milestones

In the mid-20th century, significant progress was made in isolating the individual components of the complement system, laying the foundation for understanding its biochemical structure. During the 1940s and , researchers such as Manfred Mayer developed standardized hemolytic assays that enabled the systematic fractionation of serum, leading to the identification of multiple heat-labile factors. By the late and into the , competitive efforts by groups including those led by Irwin H. Lepow resulted in the purification and characterization of the nine core components of the classical pathway, designated C1 through C9, through techniques like and . These isolations confirmed the sequential activation nature of the cascade and were pivotal in shifting research from phenomenological observations to molecular dissection. The 1950s and 1960s also saw the detailed elucidation of the classical pathway's mechanism, primarily through the work of Hans J. Müller-Eberhard and collaborators. Müller-Eberhard's group purified key proteins and demonstrated the stepwise assembly, including the formation of the as the complex C4b2a, which cleaves C3 to amplify the response. This three-component (C4b, C2a) was shown to be central to opsonization and downstream effector functions, with structural analyses revealing the proteolytic and binding domains involved. These findings integrated earlier hemolytic studies into a coherent enzymatic cascade model, influencing subsequent pathway research. The 1970s marked the discovery of the alternative pathway, expanding the complement system's activation routes beyond antibody dependence. Douglas T. Fearon and K. Frank Austen demonstrated that C3 could undergo spontaneous hydrolysis to initiate activation without classical initiators, stabilized by and amplified via factor B and factor D. was identified as a stabilizer of the (C3bBb), while factor D acted as the cleaving factor B. This pathway, building on earlier properdin system hypotheses from the , highlighted complement's role in innate immunity against microbes lacking specific antibodies. From the 1980s to the 1990s, the was identified as a third activation route, paralleling the classical pathway but triggered by carbohydrate recognition. Mannose-binding (MBL) was shown to associate with MBL-associated serine proteases (MASPs), leading to C4 and C2 cleavage and formation analogous to C4b2a. Concurrently, genetic studies mapped many complement regulators to the regulators of complement activation (RCA) locus on chromosome 1q32, including genes for CR1, CR2, , and C4-binding protein. and linkage analyses confirmed this cluster's organization, revealing evolutionary duplications and implications for regulation. In the 2000s, advances in provided atomic-level insights into complement proteins, exemplified by of C3. The 2005 structure of native C3 revealed its multi-domain architecture, including thioester-containing and anaphylatoxin domains, elucidating conformational changes upon to C3b. These visualizations clarified substrate recognition and regulatory interactions, facilitating targeted studies on pathway convergence and inhibition.

Components and

Major complement proteins

The major complement proteins are soluble components primarily synthesized by hepatocytes and circulating in plasma, where they constitute approximately 15% of the total fraction at combined concentrations exceeding 3 g/L. These proteins are classified based on their roles in initiating specific pathways or participating in the central and terminal phases of the cascade.

Classical Pathway Initiators

C1q serves as the recognition subunit of the classical pathway, a hexameric protein composed of 18 polypeptide chains arranged into six globular heads connected by collagen-like stalks, enabling binding to Fc regions of antigen-bound IgM or IgG or to certain non-antibody ligands on surfaces. C1r and C1s are homologous zymogenic serine proteases that assemble into a calcium-dependent (C1r)₂(C1s)₂ tetramer bound to C1q, forming the C1 complex. Upon , C1s cleaves C4 and C2 to form the . C4 is a 200 present at 0.2–0.4 g/L in plasma, cleaved into C4a and C4b, with C4b binding covalently to surfaces. C2, a 102 at 0.02–0.04 g/L, is cleaved by C1s into C2a and C2b; C2a associates with C4b to form the convertase. Plasma concentrations of these initiators are relatively low, with C1q at 0.12–0.22 g/L, C1r at approximately 0.03–0.04 g/L, and C1s at 0.031 g/L.

Central Component

C3 is the pivotal protein common to all three complement pathways and the most abundant complement component in plasma, with concentrations of 1–2 g/L. This 185 kDa two-chain features an exposed bond within its α-chain that becomes reactive upon proteolytic , allowing nucleophilic attack and covalent linkage to nearby or hydroxyl groups on target surfaces for enhanced opsonization.

Alternative Pathway Components

Factor B is a 93 kDa zymogenic structurally similar to C2, circulating at 0.2–0.3 g/L and serving as the substrate for the alternative pathway upon binding to C3b. Factor D, a compact 24 kDa active (lacking a zymogen form), cleaves factor B at low concentrations of 0.001–0.002 g/L. , a 53 kDa existing as oligomers, functions as the sole positive regulator of the alternative pathway by binding and stabilizing the , with plasma levels of 0.004–0.025 g/L.

Lectin Pathway Initiators

Mannose-binding lectin (MBL) is a collagenous collectin protein forming oligomeric structures that recognize neutral carbohydrate patterns (e.g., , ) on microbial surfaces, with highly variable plasma concentrations (median ~1.3 μg/mL, range <0.005–12 μg/mL) due to genetic polymorphisms. Ficolins, including ficolin-1 (M-ficolin), ficolin-2 (L-ficolin), and ficolin-3 (H-ficolin), are structurally similar to MBL with fibrinogen-like recognition domains binding acetylated groups on pathogens; plasma concentrations are ~0.005 g/L for ficolin-1, ~0.005 g/L for ficolin-2, and 0.02–0.03 g/L for ficolin-3. MASP-1 and MASP-2 are serine proteases analogous to C1r and C1s, respectively, associating with MBL or ficolins in a Ca²⁺-dependent manner to initiate the pathway; MASP-1 circulates at ~0.011 g/L, while MASP-2 is present at lower levels of ~0.0005 g/L. Like the classical pathway, activation leads to cleavage of C4 and C2.

Terminal Components

The terminal complement components C5 through C9 mediate lytic effector function by sequentially assembling into the membrane attack complex (MAC). C5, a 188 kDa disulfide-linked heterodimer at 0.07–0.2 g/L, is the initial substrate for C5 convertases. C6, C7, and C8 (concentrations ~0.04–0.08 g/L each) bind sequentially to form a pre-lytic complex that inserts into bilayers, recruiting C9 (0.058 g/L), which polymerizes into a β-barrel pore of 10–18 monomers to permeabilize target membranes.

Protein fragment nomenclature

The complement system employs a standardized for its proteins and their fragments to ensure clarity in scientific communication. The core components originating from the classical pathway are designated with the prefix "C" followed by numbers 1 through 9 (C1–C9), reflecting their order of discovery rather than activation sequence. Subcomponents of these proteins, such as those of C1, are denoted with lowercase letters (e.g., C1q, C1r, C1s). This basic numbering system was established in by an international committee to unify designations for complement components. Upon proteolytic cleavage during activation, complement proteins generate fragments that are named based on their size and position relative to the parent molecule. The smaller N-terminal fragment is typically suffixed with "a" (e.g., C3a from C3), while the larger C-terminal fragment receives the "b" suffix (e.g., C3b). Inactivated or modified forms of these fragments are indicated by a lowercase "i" prefix (e.g., iC3b, derived from further processing of C3b). This fragment notation convention was formalized in the early 1980s through recommendations by the (WHO) and the International Union of Immunological Societies (IUIS), building on earlier proposals to address inconsistencies in describing activation products. Enzyme complexes known as convertases, which cleave key complement proteins like C3 and C5, follow a composite naming system reflecting their subunit composition. The classical pathway is denoted as C4b2a (or C4bC2a), comprising the C4b fragment, C2a (the activated smaller fragment of C2), and sometimes associated with C1s. In contrast, the alternative pathway is named C3bBb, consisting of C3b and the Bb fragment from factor B. These names adhere to the same fragment rules and were standardized in a 1981 WHO-IUIS report specifically for the alternative pathway, with broader updates in 2014 by the Complement Nomenclature Committee. Among the fragments, the anaphylatoxins—C3a, C4a, and C5a—represent small peptides released from the N-terminal portions of their respective parent proteins upon cleavage by convertases or other proteases. These ~8–11 kDa molecules are potent mediators of , binding to specific receptors to induce release, contraction, and . Their directly applies the "a" and was consistently defined in the 1981 standardization efforts to distinguish them from opsonizing "b" fragments. The overall system, including these specifics, underwent its first major revision since 1981 in 2014 to incorporate newly discovered proteins and resolve ambiguities, such as those in C2 fragment designations.

Activation Pathways

Classical pathway

The classical pathway of the complement system is triggered by the binding of the C1q subcomponent to the Fc regions of immunoglobulin M (IgM) or immunoglobulin G (IgG) antibodies that are complexed with antigens on the surface of pathogens or damaged cells. This antibody-dependent recognition provides a link to the adaptive immune response, ensuring targeted activation only at sites of immune complex formation, without spontaneous initiation in the absence of antibodies. Upon binding, typically requiring at least two Fc regions for IgG or one pentameric IgM molecule, C1q undergoes a conformational change that activates the associated serine proteases C1r and C1s within the C1 complex (C1qrs₂). Activated C1r then autoactivates and cleaves C1s to its active form, enabling the cascade to proceed. The activated C1s protease subsequently cleaves the complement protein C4 into two fragments: the small anaphylatoxin C4a, which is released into the fluid phase, and the larger C4b fragment, which covalently attaches to nearby surfaces via a reactive bond, often anchoring near the immune complex. C1s also cleaves C2, a single-chain , into the smaller C2b fragment (released) and the larger C2a fragment, which binds non-covalently to the surface-bound C4b to form the classical pathway , C4b2a (also known as C4bC2a). This bimolecular complex is stabilized on the target surface and exhibits specificity for immune complexes due to the initial antibody-mediated localization of C1q. The C4b2a then proteolytically cleaves the central complement protein C3 into C3a, another anaphylatoxin that promotes , and C3b, which serves as a key by binding covalently to the surface. Surface-bound C3b can associate with additional C4b2a molecules to form the , C4b2a3b, which initiates the terminal complement cascade by cleaving C5 into C5a (an anaphylatoxin and chemoattractant) and C5b, the latter nucleating assembly of the membrane attack complex. This stepwise amplification ensures efficient deposition of complement fragments only at antibody-coated targets, enhancing pathogen clearance through opsonization and subsequent effector functions.

Alternative pathway

The alternative pathway of the complement system provides a continuous, antibody-independent mechanism for innate immune surveillance, initiated by the spontaneous of the bond within the central component C3, generating the metastable form C3(H₂O). This occurs at a low rate in plasma, altering C3's conformation to expose binding sites for factor B, a . Factor B binds to C3(H₂O), forming the proenzyme C3(H₂O)B, which is then cleaved by factor D—a circulating —into Ba and Bb fragments, yielding the fluid-phase C3(H₂O)Bb. This initial convertase cleaves additional C3 molecules into C3a (an anaphylatoxin) and C3b, establishing a basal level of known as the tickover mechanism. The tickover mechanism maintains a steady, low-level generation of C3b in the fluid phase, preventing widespread activation on host cells while enabling rapid response to ; this discrimination relies on the absence of membrane-bound regulators like () and membrane cofactor protein (MCP) on non-self surfaces, which would otherwise disassemble the convertase. Upon contact with pathogen surfaces—such as bacterial cell walls or foreign materials—C3b deposits covalently via its reactive , recruiting factor B to form the surface-bound complex C3bB. Factor D cleaves factor B in this complex, producing the active C3bBb, which amplifies C3b production exponentially by cleaving more C3. , the only known positive regulator in the pathway, binds to and stabilizes C3bBb, extending its approximately 5- to 10-fold, from ~90 seconds to 7–15 minutes and directing activation toward target surfaces. As an amplification loop, the alternative pathway generates the majority of C3b molecules during complement activation, regardless of the initiating pathway, thereby enhancing opsonization, , and membrane attack complex formation across the system; this role underscores its function as a and booster mechanism for innate immunity. The concept of "protected surfaces," where alternative pathway convertases form stably on non-host materials due to resistance to inactivation, was first elucidated in seminal work demonstrating selective activation on rabbit erythrocytes versus zymosan particles.

Lectin pathway

The lectin pathway of the complement system is initiated by the recognition of specific carbohydrate patterns on microbial surfaces through molecules, primarily mannose-binding (MBL) and ficolins. These soluble proteins circulate in plasma as complexes with mannose-binding lectin-associated serine proteases (MASPs), including MASP-1, MASP-2, and MASP-3. Upon binding to mannose or residues on pathogens, conformational changes in MBL or ficolins trigger autoactivation of MASP-1, which in turn activates MASP-2. This process is analogous to the activation of C1r and C1s in the classical pathway, reflecting structural and functional similarities among these serine proteases. Activated MASP-2 then cleaves complement component C4 into C4a and C4b fragments, followed by cleavage of C2 into C2a and C2b. The resulting C4b and C2a fragments associate to form the C4b2a, which is identical to the convertase in the classical pathway and proceeds to cleave C3, amplifying the complement response. MASP-1 enhances this efficiency by facilitating MASP-2 activation under physiological conditions, ensuring robust pathway initiation without reliance on antibodies. Variants of the initiating complexes include those formed by ficolins (H-ficolin, L-ficolin, and M-ficolin), which recognize acetylated groups on microbes and associate with the same MASPs as MBL. Additionally, collectin-11 (also known as CL-K1) functions as another recognition molecule, forming complexes with MASPs to bind pathogen-associated molecular patterns and initiate the pathway. These diverse initiators broaden the lectin pathway's ability to detect a range of microbial threats in an antibody-independent manner. Evolutionarily, the lectin pathway shares a common ancestry with the classical pathway, as evidenced by the homology between MASPs and the C1r/C1s proteases; both families belong to the C1r/C1s/sea urchin VEGF/plasminogen () superfamily, with conserved modular structures including CUB, EGF, and domains. This structural similarity underscores the pathways' parallel mechanisms for downstream activation while highlighting the pathway's role in innate immunity through direct carbohydrate recognition.

Effector Mechanisms

Membrane attack complex formation

The terminal phase of complement activation, common to all three pathways, begins with the formation of enzymes that cleave the central complement protein C5 into its fragments C5a and C5b. In the classical and pathways, the C5 convertase is the complex C4b2a3b, where C3b associates with the C4b2a to enable C5 cleavage. In the alternative pathway, the convertase is C3bBb3b, stabilized by and similarly cleaving C5. This cleavage step marks the irreversible commitment to the terminal pathway, with C5b serving as the initiating subunit for downstream assembly while C5a acts as an anaphylatoxin. Assembly of the membrane attack complex (MAC), also known as C5b-9, proceeds sequentially in the fluid phase before insertion. C5b rapidly binds C6 to form the metastable C5b6 intermediate, which then associates with C7 to generate C5b67; the latter complex exposes hydrophobic domains on C7 that facilitate binding and penetration into the of the target . C8 subsequently binds to C5b67, forming C5b-8, which further embeds into the and creates a low-affinity for C9. Multiple C9 monomers (typically 10 to 18) then polymerize onto C5b-8 in a unidirectional, manner, forming a β-barrel transmembrane channel approximately 100 (10 nm) in . This poly-C9 structure completes the MAC pore, with the number of C9 units determining the pore's size and lytic efficiency. The primary lytic mechanism of the MAC involves disruption of target cell membrane integrity, leading to uncontrolled influx of water and ions that causes colloid osmotic and . The pore's large diameter allows passage of small molecules and ions, rapidly depolarizing the and compromising cellular , particularly effective against and enveloped viruses. MAC stability is enhanced by the cylindrical arrangement of C9 monomers, which resists dissociation, although the complex can disassemble over time if not fully polymerized. In quantitative terms, complete pores with 12-18 C9 units exhibit maximal cytolytic activity, while incomplete assemblies may be less stable. Beyond , sublytic concentrations of MAC can elicit non-lytic signaling in host cells, promoting inflammatory responses without cell death. Insertion of partial MAC pores triggers calcium influx and activation of pathways such as and the , leading to production and enhanced immune cell recruitment. These effects are particularly relevant in endothelial and epithelial cells, where sublytic MAC contributes to vascular inflammation and tissue remodeling during immune responses.

Opsonization and chemotaxis

Opsonization is a key effector mechanism of the complement system, whereby activated complement proteins tag pathogens and immune complexes for enhanced recognition and uptake by phagocytic cells. The primary opsonin is C3b, generated through cleavage of C3 during complement activation, which covalently binds to the surface of microbes or altered host cells, marking them for phagocytosis. This C3b coating facilitates binding to complement receptor 1 (CR1, also known as CD35) on phagocytes such as macrophages and neutrophils, promoting efficient engulfment. Further proteolytic processing of C3b yields iC3b, which binds to complement receptor 3 (CR3, CD11b/CD18) and complement receptor 4 (CR4), extending the opsonization window and enabling phagocytosis even after initial C3b decay. In contrast, C4b, produced in the classical and lectin pathways, plays a minor opsonizing role by binding to CR1, but its contribution is less pronounced compared to C3b/iC3b due to lower deposition efficiency and rapid inactivation. Complement-mediated opsonization dramatically boosts phagocytic efficiency, often enhancing uptake by 10- to 100-fold compared to non-opsonized targets, underscoring its in innate immunity. This process not only accelerates clearance but also synergizes with antibody-mediated opsonization via Fcγ receptors, where CR1 acts as a co-receptor to lower the threshold for engulfment. For instance, C3b/iC3b-opsonized are rapidly internalized by professional , preventing dissemination and limiting . Beyond opsonization, complement fragments drive and through anaphylatoxins C3a and C5a, small peptides released upon C3 and C5 cleavage. These anaphylatoxins bind to G-protein-coupled receptors—C3a to C3aR and C5a to C5aR1 (CD88)—on immune cells including , , and neutrophils, triggering intracellular signaling via Gαi proteins. This binding induces degranulation, releasing and other mediators that increase and promote local . Concurrently, C3a and C5a act as potent chemoattractants, directing leukocyte migration to infection sites by activating chemotactic responses and enhancing adhesion molecule expression on endothelial cells. Complement also facilitates the non-inflammatory clearance of apoptotic cells, maintaining tissue homeostasis without triggering damaging responses. C1q binds directly to exposed phospholipids or altered surface molecules on apoptotic cells, initiating classical pathway activation and localized C3b deposition for opsonization. Phagocytes then recognize these opsonized cells via CR1 and CR3, leading to silent engulfment that suppresses pro-inflammatory cytokine release, such as IL-12, and promotes anti-inflammatory signals like TGF-β production. This mechanism prevents secondary necrosis and autoimmunity by efficiently removing over 10^11 apoptotic cells daily in humans.

Regulation

Soluble regulatory proteins

The soluble regulatory proteins of the complement system are circulating plasma components that prevent uncontrolled in the fluid phase, thereby limiting and tissue damage while allowing targeted responses on surfaces. These inhibitors act at various steps of the pathways, primarily by inhibiting activity, accelerating the decay of convertase enzymes, or serving as cofactors for proteolytic inactivation of complement fragments. Key examples include , , factor I, and C4-binding protein, each with distinct roles in regulating the classical, , and alternative pathways. C1 inhibitor (C1-INH), a serine protease inhibitor (serpin) family member, is the primary regulator of the classical and lectin pathways by irreversibly binding and inhibiting the activated serine proteases C1r and C1s of the C1 complex, as well as MBL-associated serine proteases (MASPs) in the lectin pathway, thereby preventing spontaneous C4 and C2 cleavage. This inhibition occurs through formation of a covalent complex that sterically blocks substrate access, maintaining complement in a quiescent state in plasma. C1-INH circulates at a plasma concentration of approximately 0.25 g/L (range: 0.15–0.35 g/L), with a half-life of 67–72 hours, and is primarily synthesized by hepatocytes. Deficiency or dysfunction of C1-INH, as seen in hereditary angioedema (HAE) types I and II, leads to unchecked bradykinin production via dysregulation of the kallikrein-kinin system and uncontrolled activation of the classical complement pathway (evidenced by low C4 and C2 levels), but the recurrent episodes of subcutaneous and mucosal edema result primarily from bradykinin-induced vascular permeability. Factor H, a 155-kDa glycoprotein composed of 20 short consensus repeats (SCRs), serves as the principal soluble regulator of the alternative pathway by binding to C3b, accelerating the decay of the C3 convertase (C3bBb) through displacement of Bb, and acting as a cofactor for factor I-mediated cleavage of C3b to iC3b. Its affinity for C3b is enhanced on host surfaces via interactions with sialic acid and glycosaminoglycans, distinguishing self from non-self. Factor H is present in plasma at concentrations of 250–600 μg/mL and is mainly liver-derived, though extrahepatic production occurs in fibroblasts and endothelial cells. Inherited deficiencies or mutations in factor H, often involving the C-terminal SCRs 19–20, are strongly associated with atypical hemolytic uremic syndrome (aHUS) and age-related macular degeneration, where impaired regulation leads to excessive C3 activation and endothelial damage. Factor I, a soluble of approximately 88 kDa, functions as the central inactivator of complement by proteolytically cleaving C3b to iC3b and further to C3dg, as well as C4b to C4d, but only in the presence of cofactors such as or C4-binding protein. This cofactor-dependent activity limits the amplification loop of the alternative pathway and inhibits classical/ convertases, preventing widespread opsonization in plasma. Plasma levels of factor I are typically 20–50 μg/mL, with synthesis occurring in the liver and monocytes. Complete or partial deficiencies in factor I result in uncontrolled C3 consumption, recurrent pyogenic infections (e.g., with species), and susceptibility to autoimmune conditions like systemic , as the lack of inactivation allows persistent complement activation. C4-binding protein (C4BP), a large oligomeric (molecular mass ~570 kDa) consisting of seven α-chains and one β-chain linked to , regulates the classical and pathways by binding C4b, dissociating the (C4b2a), and serving as a cofactor for factor I-mediated degradation of C4b. This action curtails C3 activation downstream of C1-INH, particularly in fluid phase where C4BP concentrations exceed those of C4b. C4BP circulates at ~200 μg/mL (range: 150–300 μg/mL) and is predominantly hepatic in origin, with levels increasing as an acute-phase reactant. Although rare, C4BP deficiency has been linked to increased autoimmune manifestations, such as and , due to dysregulated classical pathway activity, though it often co-occurs with alterations affecting .

Membrane-associated regulators

Membrane-associated regulators of the complement system are cell surface proteins that locally inhibit complement activation to protect host tissues from inadvertent damage during immune responses. These proteins, anchored to the plasma membrane via transmembrane domains or glycosylphosphatidylinositol (GPI) linkages, act at various stages of the complement cascade, primarily on self cells to prevent amplification of C3 convertases, degradation of opsonins, and assembly of the membrane attack complex (MAC). By restricting complement activity to pathogen surfaces or altered self cells that express lower levels of these regulators, they contribute to self/non-self discrimination, ensuring that autologous cells are spared while foreign entities are targeted. Decay-accelerating factor (DAF, CD55) is a GPI-anchored expressed on the surface of most cells, including erythrocytes, leukocytes, endothelial cells, and epithelial cells. It accelerates the decay of classical and alternative pathway C3 and C5 convertases by binding to and dissociating their components, thereby limiting C3b deposition and downstream effector functions on host membranes. This protective mechanism is crucial for preventing complement-mediated of self cells in contact with serum. Membrane cofactor protein (MCP, CD46) is a ubiquitously distributed on nucleated cells, such as endothelial cells, fibroblasts, and leukocytes, but absent on erythrocytes. It serves as a cofactor for the factor I, facilitating the proteolytic cleavage and inactivation of C3b and C4b deposited on cell surfaces, which halts the amplification of complement in both classical and alternative pathways. MCP's broad tissue expression ensures targeted regulation at sites of potential complement exposure. Complement receptor 1 (CR1, CD35) is a transmembrane predominantly found on erythrocytes, monocytes, neutrophils, and B cells, with lower expression on other leukocytes and tissue cells. It combines decay-accelerating activity, similar to , by promoting the dissociation of C3 and C5 convertases, and cofactor activity for factor I-mediated cleavage of C3b and C4b. On erythrocytes, CR1 plays a key role in immune complex clearance while protecting the carrier cells from complement damage. Protectin (CD59) is a GPI-anchored protein widely expressed on human cells, including hematopoietic cells, endothelial cells, and epithelial surfaces. It binds to the partially formed C5b-8 complex and inhibits the recruitment and polymerization of C9, thereby preventing the formation of the pore-forming MAC on host cell membranes. This terminal pathway inhibition provides a final safeguard against complement-mediated cytolysis. Collectively, these regulators are constitutively expressed at varying densities on host cells, with higher levels on cells frequently exposed to complement, such as and vascular elements. Their absence or downregulation on pathogens or stressed self cells allows unchecked complement activation, facilitating immune discrimination between self and non-self. Seminal studies on their structures and functions, such as those elucidating DAF's decay mechanism and CD59's MAC inhibition, have established their essential roles in complement .

Physiological Roles

Role in innate immunity

The complement system serves as a cornerstone of innate immunity by providing rapid, antibody-independent defense against through direct recognition, activation of effector mechanisms, and coordination of inflammatory responses. In this capacity, it operates primarily via the alternative and pathways, which enable surveillance and elimination of invading microbes without prior sensitization. These pathways initiate a proteolytic cascade that converges on the central component C3, leading to opsonization, inflammation, and cytolytic activity, thereby bridging pathogen detection with immune clearance. A key aspect of the complement system's innate role is its direct antimicrobial activity, achieved through formation of the membrane attack complex (MAC) that lyses susceptible . In the alternative pathway, spontaneous of C3 exposes a bond, allowing low-level deposition on surfaces and amplification via factor B and D, culminating in MAC assembly (C5b-9) that perforates bacterial membranes. For instance, this pathway is critical for lysing such as Neisseria meningitidis, where complement activation on the bacterial surface drives rapid killing in serum, as evidenced by heightened susceptibility of capsule-deficient mutants to alternative pathway-mediated . The complements this by recognizing microbial glycans through pattern recognition molecules like mannose-binding (MBL) and ficolins, which bind motifs on surfaces and trigger MASP-mediated C4 and C2 cleavage, leading to C3 activation and MAC formation. This mechanism contributes to the of enveloped viruses, such as , by targeting viral glycoproteins, and certain parasites, including Trypanosoma cruzi, where lectin-initiated MAC insertion disrupts parasite membranes and limits infection propagation. Beyond lysis, the complement system facilitates innate immune surveillance by recruiting to sites. Cleavage of C5 during pathway activation generates C5a, a potent anaphylatoxin that acts as a chemoattractant for neutrophils, promoting their migration across endothelial barriers via C5a receptor (C5aR1) signaling. This enhances engulfment and clearance at localized inflammatory foci, amplifying the innate response without adaptive involvement. The lectin pathway's further underscores this surveillance function, as MBL and ficolins detect evolutionarily conserved microbial glycans—such as mannose-rich structures on bacterial lipopolysaccharides or fungal mannans—that differ from host sialylated glycans, thereby distinguishing self from non-self and initiating targeted complement deposition. The complement system's innate functions reflect its evolutionary conservation, predating adaptive immunity and extending to . Homologs of C3, the pivotal and anaphylatoxin precursor, have been identified in non-vertebrate deuterostomes, including the purple sea urchin (Strongylocentrotus purpuratus), where C3-like proteins in coelomocytes mediate primitive proteolytic cascades for defense. This ancient presence, traceable to early metazoans like sponges, highlights complement's role as a foundational innate mechanism, with expansions in vertebrates enhancing its efficiency.

Interaction with adaptive immunity

The complement system bridges innate and adaptive immunity by enhancing B-cell activation through the deposition of C3 fragments on antigens. Specifically, C3d-tagged antigens bind to (CR2, also known as CD21) on B cells, which co-ligates with the (BCR) via the CD19/CD81 complex, delivering a costimulatory signal that amplifies BCR signaling 10- to 100-fold and lowers the activation threshold. This mechanism facilitates more efficient antigen-specific B-cell responses, particularly for low-avidity interactions, and is crucial for . Antibodies provide feedback to the complement system primarily via the classical pathway, where IgM and certain IgG subclasses (IgG1 and IgG3 in humans) bind s and recruit C1q to initiate complement activation. This leads to C3b and C3d deposition on immune complexes, which not only promotes opsonization but also supports B-cell affinity maturation in germinal centers by facilitating antigen delivery to (FDCs) and B cells. Complement-mediated enhancement of antibody responses can increase titers by 10- to 1000-fold in model systems, underscoring its role in amplifying adaptive . Complement anaphylatoxins C3a and C5a further integrate with cellular adaptive responses by modulating (DC) function and T-cell differentiation. C3a and C5a, generated locally during immune activation, bind to receptors on DCs (C3aR and C5aR), promoting their maturation, production (e.g., IL-12), and migration to nodes, which enhances priming of naive T cells. These signals also influence T-helper cell polarization, with C5a favoring Th1 responses through DC activation while C3a can promote Th2 skewing in certain contexts, thereby fine-tuning adaptive immunity. In long-term adaptive immunity, complement contributes to immunological by aiding the clearance of immune complexes via CR1 (CD35) on erythrocytes and FDCs, which prevents excessive and sustains for B-cell maintenance. This process ensures efficient removal of circulating complexes while preserving depots on FDCs for secondary responses.

Pathophysiological Roles

Complement deficiencies

Complement deficiencies refer to inherited defects in the proteins of the complement system, which impair its and function, leading to increased susceptibility to infections and, in some cases, autoimmune conditions. These deficiencies are primarily genetic and affect specific components of the classical, alternative, or common pathways, resulting in recurrent bacterial infections due to compromised opsonization, , or membrane attack complex (MAC) formation. Most complement deficiencies follow an autosomal recessive inheritance pattern, requiring biallelic mutations for clinical manifestation, though exceptions like properdin deficiency are X-linked. The prevalence varies by component and population; for instance, C2 deficiency occurs in approximately 1 in 20,000 individuals of Caucasian descent, making it one of the more common forms. Deficiencies in the terminal components (C5 through C9) disrupt MAC formation, severely impairing the lytic killing of certain while leaving upstream functions like opsonization intact. Affected individuals face a dramatically elevated risk of invasive infections by species, such as , with the risk increased up to 10,000-fold compared to the general population. These patients often experience recurrent , though other infections are less frequent due to preserved early pathway activities. Early classical pathway deficiencies, involving C1, C4, or C2, compromise the initiation of complement activation triggered by immune complexes or antibodies. This leads to inefficient clearance of apoptotic cells and immune complexes, predisposing individuals to systemic lupus erythematosus (SLE)-like characterized by autoantibodies, , and . In addition to autoimmune risks, these deficiencies increase vulnerability to infections with encapsulated bacteria, though less severely than central pathway defects. Alternative pathway deficiencies, particularly in properdin or factor D, hinder the amplification loop that sustains complement activation on microbial surfaces. stabilizes the , and its absence results in fulminant meningococcal infections with high mortality, often presenting as severe or . Factor D deficiency similarly impairs alternative pathway initiation, leading to recurrent Neisseria infections, though cases are rarer and inheritance is autosomal recessive for factor D but X-linked for properdin. C3 deficiency represents a critical bottleneck, as C3 is central to all complement pathways and essential for opsonization, , and downstream effector functions. Complete C3 deficiency causes severe, recurrent pyogenic infections starting in early childhood, predominantly by and species, including , , and due to failed amplification and bacterial clearance. These patients may also develop immune complex-mediated diseases, but infections dominate the clinical picture.

Involvement in diseases

The complement system contributes to pathology in various diseases through excessive activation or dysregulation, leading to tissue damage, inflammation, and immune-mediated injury. In autoimmune conditions, dysregulated complement components such as C3 and C5a exacerbate disease progression by amplifying inflammatory responses triggered by immune complexes. For instance, in rheumatoid arthritis (RA), immune complexes containing IgG deposit in synovial tissues, activating the classical pathway and resulting in C3 deposition in over 90% of RA synovial fluids, which promotes chronic joint inflammation and cartilage destruction. Similarly, in systemic lupus erythematosus (SLE), immune complexes activate the classical complement pathway via C1q binding, generating C3 fragments and C5a that drive neutrophil recruitment and tissue injury in organs like the kidneys and skin, contributing to lupus nephritis and vasculitis. Atypical hemolytic uremic syndrome (aHUS) exemplifies complement dysregulation leading to vascular pathology, where mutations in complement (CFH) impair alternative pathway regulation, causing uncontrolled activity and excessive endothelial cell damage. These CFH mutations, often loss-of-function variants, reduce the protein's ability to bind and inactivate C3b on host surfaces, resulting in persistent complement activation, , , and through endothelial lysis and thrombus formation. Paroxysmal nocturnal hemoglobinuria (PNH) involves complement-mediated due to deficiencies in membrane regulators CD55 and , rendering red blood cells highly susceptible to the membrane attack complex (MAC). The absence of CD55, which accelerates decay of C3 and C5 convertases, combined with deficiency, which inhibits MAC assembly, leads to chronic intravascular , nocturnal hemoglobinuria, and increased risk as affected erythrocytes are lysed by alternative and classical pathway activation in the bloodstream. In age-related macular degeneration (), chronic complement activation in the contributes to formation and photoreceptor degeneration, with variants in the CFH gene serving as major risk factors. , extracellular deposits beneath the , contain complement proteins including C3 and C5b-9 (MAC), indicating local alternative pathway overactivation that promotes and choroidal in late-stage ; the common Y402H polymorphism in CFH reduces its regulatory function, heightening susceptibility to this degenerative process. Recent studies have linked complement hyperactivation to the observed in severe cases, where infection triggers excessive classical and alternative pathway activation, amplifying proinflammatory responses. Post-2020 analyses show elevated plasma levels of complement activation products like C5a and C3a in critically ill patients, correlating with , , and multi-organ failure as these anaphylatoxins recruit neutrophils and exacerbate the hyperinflammatory state.

Diagnostic and therapeutic approaches

Diagnosis of complement system disorders relies on functional assays that evaluate pathway activity and specific protein quantification to identify deficiencies or dysregulation. The CH50 assay measures total hemolytic complement activity in the classical pathway by assessing the ability of patient serum to lyse antibody-sensitized sheep erythrocytes, serving as a screening tool for deficiencies in classical pathway components from C1 to C9. Similarly, the AH50 assay evaluates alternative pathway function through of unsensitized rabbit erythrocytes, detecting abnormalities in factors B, D, , or C3. These hemolytic assays provide an overall assessment of pathway integrity but may miss isolated regulator defects. For precise identification of individual component levels, enzyme-linked immunosorbent assays (ELISAs) are employed, such as those quantifying C3 concentrations in serum, which help confirm hereditary or acquired deficiencies associated with recurrent infections or autoimmune conditions. Functional assays for regulators, including cofactor activity tests for factor I or decay acceleration assays for , utilize ELISA-based detection of C3b deposition or MAC formation to assess inhibitory protein efficacy. These methods enable targeted diagnosis, particularly in (aHUS) or (PNH), where pathway dysregulation predominates. Therapeutic modulation of the complement system primarily involves inhibitors targeting key activation points to mitigate excessive activity in diseases like PNH and aHUS. , a against C5, was approved by the FDA in 2007 for reducing in PNH and in 2011 for aHUS, preventing C5a and MAC formation. , a longer-acting C5 inhibitor with an extended half-life, received FDA approval in 2018 for PNH and later for aHUS, allowing less frequent dosing while maintaining efficacy. Proximal inhibition strategies include , a pegylated C3 inhibitor approved by the FDA in 2021 for PNH, which blocks to address both intravascular and extravascular . Emerging therapies target alternative pathway amplifiers, such as danicopan, an oral factor D inhibitor approved by the FDA in 2024 as add-on therapy to C5 inhibitors for PNH patients with persistent extravascular hemolysis, enhancing hemoglobin levels in clinical trials. For complement deficiencies, gene therapy approaches are in preclinical development; for instance, adeno-associated virus (AAV)-mediated delivery of complement factor H has shown promise in resolving C3 glomerulopathy models by restoring regulation. Recent 2025 studies on truncated CFH AAV therapy have demonstrated long-term disease reversal in models. These strategies aim to provide durable correction, though clinical translation remains challenged by immune responses to vectors.

Emerging and Specialized Roles

Role in the central nervous system

The complement system contributes to brain development by facilitating , a process essential for refining neural circuits. During postnatal development, C1q, the initiating component of the , is expressed by neurons in response to signals from immature and localizes to developing s, particularly in the retinogeniculate pathway critical for maturation. This leads to the deposition of C3b opsonins on synapses, marking them for recognition and by via complement receptor 3 (CR3). Deficiency in C1q or C3 results in impaired synapse elimination, with mice exhibiting persistent multiple innervation of axons onto thalamocortical neurons, underscoring complement's necessity for proper circuit refinement. Complement activity in the (CNS) is largely independent of systemic circulation due to the blood-brain barrier (BBB), which restricts serum protein leakage. Instead, complement components are synthesized locally by CNS-resident cells, primarily and , enabling rapid responses to developmental and homeostatic needs. produce key proteins such as C3 and C4, while express terminal pathway components like C5-C9, supporting opsonization and phagocytic clearance without relying on peripheral sources. This localized synthesis maintains low baseline complement levels in the healthy while allowing upregulation during physiological remodeling. In neurodegenerative contexts, complement fragments C3a and C5a drive , exacerbating disease progression. In (MS), C5a and C3a anaphylatoxins activate and in cortical lesions, promoting proinflammatory release, blood-brain barrier disruption, and demyelination through classical and alternative pathway activation. Similarly, in (AD), these fragments amplify microglial responses around amyloid-β plaques, enhancing plaque compaction and associated synaptic loss while contributing to chronic . Complement opsonization of plaques, initiated by C1q, tags them for clearance but often leads to excessive glial activation and neurodegeneration when dysregulated. Preclinical studies in the 2020s highlight therapeutic potential for modulating complement in CNS disorders. C5aR antagonists, such as PMX205, reduce C5a-mediated microglial polarization toward proinflammatory states, decreasing pathology, , and cognitive deficits in AD mouse models. In neuroinflammatory models of hypoxia-ischemia and AD, these antagonists preserve synaptic integrity and mitigate plaque-associated without broadly impairing protective complement functions. Ongoing research emphasizes their ability to cross the BBB, offering targeted intervention for complement-driven neurodegeneration.

Non-immune functions and evolution

Beyond its classical roles in immunity, the complement system contributes to various non-immune processes essential for tissue and development. In , C1q plays a critical protective role in trophoblast function and embryo implantation by facilitating the migration of extravillous trophoblasts into the and promoting spiral artery remodeling, which ensures adequate placental blood flow. This local production of C1q by decidual endothelial cells and trophoblasts helps maintain feto-maternal tolerance without triggering inflammatory responses. Complement components also support wound healing through angiogenic mechanisms. The anaphylatoxin C3a, generated during complement activation, stimulates endothelial cell proliferation and vascular tube formation, thereby enhancing neovascularization at injury sites to promote tissue repair. This process is particularly evident in models of cutaneous , where C3a signaling recruits pro-angiogenic cells and modulates the to accelerate closure. In metabolic regulation, complement factor C3, produced by adipocytes, links adipose tissue dysfunction to insulin resistance and obesity. Elevated C3 levels in obese individuals correlate with impaired glucose uptake and hepatic steatosis, as adipocyte-derived C3 activates local inflammation and disrupts insulin signaling pathways. Recent studies further highlight C3's involvement in cancer metabolism, where activation in tumor microenvironments promotes lipid reprogramming that fuels tumor growth independently of immune surveillance. Emerging has identified intracellular complement activation, termed the "complosome," as a key non-immune function regulating cellular processes such as , , and mitochondrial function in various tissues, with implications for diseases like cancer and neurodegeneration as of 2023. The complement system represents an ancient phylogenetic , with origins tracing back to the emergence of multicellular animals over 500 million years ago. Homologs of C3, the central component, have been identified in cnidarians such as , indicating that a primitive thioester-containing protein capable of opsonization and existed in these early metazoans. Unlike vertebrates, lower lack the membrane attack complex (MAC), relying instead on alternative pathways involving C3-like molecules and factor B for recognition and clearance. Adaptive radiation of the complement system occurred in vertebrates, where duplications expanded the classical, , and alternative pathways, enabling more sophisticated immune and non-immune functions. This diversification, evident from jawed vertebrates onward, integrated complement with adaptive immunity while preserving ancestral roles in development and .

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