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Antibodies, also called immunoglobulins, are Y-shaped glycoprotein molecules produced mainly by plasma cells derived from B lymphocytes. They are a central component of the humoral immune response and bind with high specificity to antigens such as pathogens, toxins, or foreign particles.

An antibody molecule consists of two identical heavy chains and two identical light chains linked by disulfide bonds. Each chain has constant and variable regions; the variable regions from the two chains form the antigen-binding site (paratope) that recognizes a specific epitope. The constant region (Fc) mediates effector functions.

Diversity arises from V(D)J gene rearrangements during B cell development, junctional diversity, and somatic hypermutation in germinal centers. After activation, B cells differentiate into plasma cells that secrete soluble antibodies and into memory cells for faster responses upon re-exposure.

There are five major isotypes in humans: IgG, IgA, IgM, IgD, and IgE, each with distinct distribution and roles. IgG is predominant in the blood; IgA protects mucosal surfaces; IgM is the first responder; IgD mainly serves as a B cell receptor; IgE is involved in allergy and parasite defense.

Antibodies perform several functions, including neutralization of toxins and pathogens, opsonization to enhance phagocytosis, activation of the classical complement pathway (especially IgM and IgG), and antibody-dependent cellular cytotoxicity via natural killer cells.

Clinically, antibodies underpin serology tests and vaccine-induced immunity. Monoclonal antibodies are used as therapies, autoantibodies are implicated in autoimmune diseases, and deficiencies in antibody production can cause immune disorders.