Anticorps

Anticorps, or antibodies, are soluble glycoproteins produced primarily by plasma cells derived from B lymphocytes in vertebrate immune systems. They recognize and bind specifically to antigens, such as components of pathogens or toxins, with the aim of neutralizing the threat and coordinating other immune responses. Anticorps circulate in blood and bodily fluids and also function at mucosal surfaces in some subclasses.

Structure and classes: An antibody molecule is a Y-shaped immunoglobulin composed of two identical heavy chains and two identical light chains, linked by disulfide bonds. The tips of the two arms form the antigen-binding sites, created by the variable regions of the chains, while the stem region, the constant domains, mediates effector functions through interactions with Fc receptors on immune cells and with components of the complement system. Humans express five major classes: IgG, IgA, IgM, IgD, and IgE, each with distinct distributions and roles in immune defense. A single B cell can produce antibodies of one specificity but may switch class to alter effector function without changing antigen recognition.

Generation and actions: Antibody diversity arises from recombination of gene segments (V(D)J), junctional diversity, and pairing of heavy and light chains, followed by somatic hypermutation and, in some cases, class switch recombination during immune responses. Mechanisms of action include neutralization of pathogens or toxins, agglutination or precipitation of antigens, opsonization to promote phagocytosis, activation of the classical complement pathway, and antibody-dependent cellular cytotoxicity mediated by natural killer cells.

Clinical relevance: Antibodies underpin many diagnostic tests and therapies. Monoclonal and engineered antibodies are used to treat diseases, while serology and immunoassays detect antibodies as biomarkers of exposure or vaccination.