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Antibodies, or immunoglobulins, are large Y-shaped glycoproteins produced mainly by plasma cells derived from B lymphocytes. They circulate in blood, lymph, and many body fluids, and are also found on mucosal surfaces. Each antibody consists of two identical heavy chains and two identical light chains linked by disulfide bonds, forming a Fab region that binds antigens and a constant Fc region that interacts with other immune components.

Antibodies recognize specific antigens through their variable regions. Binding can neutralize pathogens, block toxin activity, or prevent infection. The antibody–antigen complex can also promote phagocytosis (opsonization), recruit complement to destroy targets, or mediate antibody-dependent cellular cytotoxicity.

Diversity arises from gene rearrangement, somatic hypermutation, and class switching. Humans have five isotypes: IgM, IgD, IgG, IgA, and IgE, each serving different roles and locations. Naive B cells express membrane Ig as a receptor, while activated B cells can switch isotypes and undergo affinity maturation in germinal centers to improve binding.

Clinically, antibodies provide passive immunity to the fetus and newborn via maternal IgG. Therapeutically, monoclonal antibodies treat infections and cancer, and antibodies are central to many diagnostic tests.

Research into antibody structure and function informs vaccine design, autoimmune disease studies, and biotechnological applications. Abnormal antibody responses, such as autoantibodies or allergies, reflect breakdowns in immune tolerance.