antibody

Antibodies, also called immunoglobulins, are large glycoprotein molecules produced mainly by plasma cells as part of the adaptive immune response. They circulate in blood and other body fluids and bind specifically to antigens, which are substances detected as foreign by the immune system. The binding can neutralize pathogens directly or mark them for attack by other parts of the immune system.

Antibodies have a characteristic Y-shaped structure composed of two identical heavy chains and two identical light chains, linked by disulfide bonds. Each antibody has Fab (fragment antigen-binding) regions that form the antigen-binding sites, and an Fc (fragment crystallizable) region that interacts with immune cells and complement proteins. The variable regions, including hypervariable loops, confer antigen specificity, while the constant regions determine effector functions. There are five main isotypes in humans: IgA, IgD, IgE, IgG, and IgM, each with distinct roles and distributions.

Functions of antibodies include neutralizing toxins and pathogens, agglutinating antigens, and opsonizing targets to enhance phagocytosis. The Fc region can recruit other components of the immune system, such as complement and natural killer cells, to mediate responses like antibody-dependent cellular cytotoxicity. B cells initially express membrane-bound antibodies as receptors and later secrete soluble forms into circulation or mucosal secretions; class switching and somatic hypermutation diversify antibody responses.

Applications include monoclonal antibodies, produced from a single B cell clone for high specificity, and polyclonal antibodies, derived from multiple clones. Therapeutic antibodies are used in cancer and autoimmune diseases, while diagnostic tools such as ELISA and Western blot rely on antibody–antigen interactions.