Antibodies
Y-shaped proteins produced by B cells that recognize specific antigens and drive the adaptive immune response.
Antibodies, also called immunoglobulins, are Y-shaped proteins produced by B lymphocytes that recognize and bind specific antigens — molecules from pathogens, toxins, or other foreign material. Each antibody carries two identical antigen-binding sites at the tips of the Y, formed by the variable regions of its heavy and light chains; the constant region determines the antibody's class and its effector functions.
Five classes exist in humans. IgG is the main antibody of the blood, crosses the placenta, and opsonizes pathogens; IgM is a pentamer and the first class produced in a primary response; IgA protects mucosal surfaces and appears in secretions; IgE is involved in allergy and defense against parasites; IgD is found on the surface of naive B cells. The enormous diversity of antibody specificities — billions of distinct binding sites — is generated by V(D)J recombination, the random rearrangement of gene segments during B-cell development, refined by somatic hypermutation and affinity maturation in germinal centers.
Antibodies protect through several mechanisms: neutralization (blocking a pathogen's ability to infect), opsonization (marking pathogens for phagocytosis), activation of the complement system, and antibody-dependent cellular cytotoxicity. Because the adaptive response is specific and persists, memory B cells produce a rapid secondary response on re-exposure — the principle behind vaccination.
Monoclonal antibodies — identical antibodies from a single clone, made possible by the hybridoma technique of Köhler and Milstein (1975) — are among the most successful classes of therapeutic drugs, used for autoimmune diseases, cancer (rituximab, trastuzumab), and infections, and they power familiar diagnostics such as the home pregnancy test. When antibodies mistakenly target the body's own tissues, the result is autoimmune disease.
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antibodies immune system immunology medicine