Polyclonal B cell response

Polyclonal B cell response is a natural mode of immune response exhibited by the adaptive immune system of mammals. It ensures that a single antigen is recognized and attacked through its overlapping parts, called epitopes, by multiple clones of B cell.

In the course of normal immune response, parts of pathogens (e.g. bacteria) are recognized by the immune system as foreign (non-self), and eliminated or effectively neutralized to reduce their potential damage. Such a recognizable substance is called an antigen. The immune system may respond in multiple ways to an antigen; a key feature of this response is the production of antibodies by B cells (or B lymphocytes) involving an arm of the immune system known as humoral immunity. The antibodies are soluble and do not require direct cell-to-cell contact between the pathogen and the B-cell to function.

Antigens can be large and complex substances, and any single antibody can only bind to a small, specific area on the antigen. Consequently, an effective immune response often involves the production of many different antibodies by many different B cells against the same antigen. Hence the term "polyclonal", which derives from the words poly, meaning many, and clones from Greek klōn, meaning sprout or twig; a clone is a group of cells arising from a common "mother" cell. The antibodies thus produced in a polyclonal response are known as polyclonal antibodies. The heterogeneous polyclonal antibodies are distinct from monoclonal antibody molecules, which are identical and react against a single epitope only, i.e., are more specific.

Although the polyclonal response confers advantages on the immune system, in particular, greater probability of reacting against pathogens, it also increases chances of developing certain autoimmune diseases resulting from the reaction of the immune system against native molecules produced within the host.

Diseases which can be transmitted from one organism to another are known as infectious diseases, and the causative biological agent involved is known as a pathogen. The process by which the pathogen is introduced into the body is known as inoculation, and the organism it affects is known as a biological host. When the pathogen establishes itself in a step known as colonization, it can result in an infection, consequently harming the host directly or through the harmful substances called toxins it can produce. This results in the various symptoms and signs characteristic of an infectious disease like pneumonia or diphtheria.

Countering the various infectious diseases is very important for the survival of the susceptible organism, in particular, and the species, in general. This is achieved by the host by eliminating the pathogen and its toxins or rendering them nonfunctional. The collection of various cells, tissues and organs that specializes in protecting the body against infections is known as the immune system. The immune system accomplishes this through direct contact of certain white blood cells with the invading pathogen involving an arm of the immune system known as the cell-mediated immunity, or by producing substances that move to sites distant from where they are produced, "seek" the disease-causing cells and toxins by specifically binding with them, and neutralize them in the process–known as the humoral arm of the immune system. Such substances are known as soluble antibodies and perform important functions in countering infections.

Antibodies serve various functions in protecting the host against the pathogen. Their soluble forms which carry out these functions are produced by plasma B cells, a type of white blood cell. This production is tightly regulated and requires the activation of B cells by activated T cells (another type of white blood cell), which is a sequential procedure. The major steps involved are:

Pathogens synthesize proteins that can serve as "recognizable" antigens; they may express the molecules on their surface or release them into the surroundings (body fluids). What makes these substances recognizable is that they bind very specifically and somewhat strongly to certain host proteins called antibodies. The same antibodies can be anchored to the surface of cells of the immune system, in which case they serve as receptors, or they can be secreted in the blood, known as soluble antibodies. On a molecular scale, the proteins are relatively large, so they cannot be recognized as a whole; instead, their segments, called epitopes, can be recognized. An epitope comes in contact with a very small region (of 15–22 amino acids) of the antibody molecule; this region is known as the paratope. In the immune system, membrane-bound antibodies are the B-cell receptor (BCR). Also, while the T-cell receptor is not biochemically classified as an antibody, it serves a similar function in that it specifically binds to epitopes complexed with major histocompatibility complex (MHC) molecules. The binding between a paratope and its corresponding antigen is very specific, owing to its structure, and is guided by various noncovalent bonds, not unlike the pairing of other types of ligands (any atom, ion or molecule that binds with any receptor with at least some degree of specificity and strength). The specificity of binding does not arise out of a rigid lock and key type of interaction, but rather requires both the paratope and the epitope to undergo slight conformational changes in each other's presence.