An antibody, or immunoglobulin, is a Y‑shaped protein produced by B cells of the immune system. Antibodies bind specifically to antigens to neutralise them or mark them for destruction.
Explanation
Antibodies are glycoproteins composed of two identical heavy chains and two identical light chains linked by disulfide bonds. The ends of the arms form variable regions that recognise specific antigen epitopes, while the stem and constant regions mediate biological functions. When a naive B cell encounters its matching antigen, it differentiates into a plasma cell that secretes large amounts of antibody. Antibodies protect the body in several ways: they neutralise toxins and viruses by blocking their attachment to host cells, coat pathogens to enhance phagocytosis (opsonisation) and activate the complement cascade. There are five major antibody classes, each with a distinct constant region and role. Immunoglobulin M (IgM) is the first antibody produced in an immune response and circulates as a pentamer. IgG is the most abundant class in blood and provides long‑term immunity. IgA is found in secretions such as saliva, tears and breast milk, where it guards mucosal surfaces. IgE binds to receptors on mast cells and basophils and is involved in allergic reactions and defence against parasites. IgD is present on naive B cells and helps initiate activation. Antibodies are highly specific, and the diversity of their variable regions is generated by genetic recombination and mutation.
Examples and Applications
Maternal IgG antibodies cross the placenta to protect newborns during the first months of life. Secretory IgA in breast milk helps protect infants against intestinal infections. Monoclonal antibodies are engineered molecules used to treat diseases such as rheumatoid arthritis, cancers and COVID‑19. Serological tests like ELISA detect antibodies in blood to diagnose infections or measure immune responses after vaccination.
Antibodies play a central role in humoral immunity and are widely used in medicine and research for diagnostics and therapy.