Foam cells, also called lipid-laden macrophages, are a type of cell that contain cholesterol. These can form a plaque that can lead to atherosclerosis and trigger myocardial infarction and stroke.

Foam cells are fat-laden cells with an M2 macrophage-like phenotype. They contain low density lipoproteins (LDL) and can be rapidly detected by examining a fatty plaque under a microscope after it is removed from the body. They are named because the lipoproteins give the cell a foamy appearance.

Despite the connection with cardiovascular diseases they might not be inherently dangerous.

Some foam cells are derived from smooth muscle cells and present a limited macrophage-like phenotype.

Foam cell formation is triggered by a number of factors including the uncontrolled uptake of modified low density lipoproteins (LDL), the upregulation of cholesterol esterification and the impairment of mechanisms associated with cholesterol release. Foam cells are a significant component of atherosclerotic lesions, which are formed when circulating monocyte-derived cells are recruited to the atherosclerotic lesion site or fat deposits in the blood vessel walls. Recruitment is facilitated by the molecules P-selectin and E-selectin, intercellular adhesion molecule 1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1).

In response to the inflammatory recruitment signals, monocytes are able to penetrate the arterial wall through transendothelial migration, as they can even in healthy arteries. Once in the sub endothelium space, inflammation processes induce the differentiation of monocytes into mature macrophages. Macrophages are then able to internalize modified lipoproteins like βVLDL (beta very low density lipoprotein), AcLDL (acetylated low density lipoprotein) and OxLDL (oxidized low density lipoprotein) through their binding to the scavenger receptors (SRs) such as CD36 and SR-A on the macrophage surface. These scavenger receptors act as pattern recognition receptors (PRRs) on macrophages and are responsible for recognizing and binding to oxLDL, which in turn promotes the formation of foam cells through internalization of these lipoproteins.

Coated-pit endocytosis, phagocytosis and pinocytosis are also responsible for lipoprotein internalization. Once internalized, scavenged lipoproteins are transported to endosomes or lysosomes for degradation, whereby the cholesteryl esters (CE) are hydrolyzed to unesterified free cholesterol (FC) by lysosomal acid lipase (LPL). Free cholesterol is transported to the endoplasmic reticulum where it is re-esterified by ACAT1 (acyl-CoA: cholesterol acyltransferase 1) and subsequently stored as cytoplasmic lipid droplets. These droplets are responsible for the foamy appearance of the macrophage and thus the name of foam cells. At this point, foam cells can either be degraded though the de-esterification and secretion of cholesterol, or can further promote foam cell development and plaque formation – a process that is dependent on the balance of free cholesterol and esterified cholesterol.

Low-density lipoprotein (LDL) cholesterol (LDL-C — also known as "bad" cholesterol) and particularly modified forms of LDL cholesterol such as oxidized, glycated, or acetylated LDL, is contained by a foam cell - a marker of atherosclerosis. The uptake of LDL-C alone does not cause foam cell formation; however, the co-internalization of LDL-C with modified LDL in macrophages can result in foam cell development. Modified LDL affects the intracellular trafficking and metabolism of native LDL, such that not all LDL need to be modified for foam cell formation when LDL levels are high.