Antibody

Antibodies, also called immunoglobulins (Ig), are proteins of the adaptive immune system found in blood and body fluids. Each is built from two identical heavy chains and two identical light chains, linked by disulfide bonds into a Y shape. The tips of the two arms are variable regions that form the antigen-binding sites, while the stem, the Fc region, is constant and connects to immune cells. A B cell first displays its antibody on its surface as a receptor; once activated, its descendants, plasma cells, secrete it in large amounts.

Antibodies protect in several ways: they neutralize viruses and toxins by blocking them, coat microbes so that phagocytes can engulf them (opsonization) and trigger the complement system. Humans make five classes. IgM appears first and has ten binding sites; IgG is the most plentiful in blood and crosses the placenta; IgA guards mucous membranes and is found in breast milk; IgE is linked to allergies and defense against worms; and IgD sits on naive B cells.

Antibodies persist after infection or vaccination, giving lasting protection, and their precise binding makes them key tools in tests such as ELISA.