Any of a diverse group of plasma polypeptides that bind antigenic proteins and serve as one of the body’s primary defenses against disease. Two different forms exist. The first group of immunoglobulins lies on the surface of mature B cells, enabling them to bind to thousands of antigens. When the antigens are bound, the B plasma cells secrete the second type of immunoglobulins, antigen-specific antibodies, which circulate in the blood and accumulate in lymphoid tissue, esp. the spleen and lymph nodes, binding and destroying specific foreign antigens and stimulating other immune activity. Antibodies also activate the complement cascade, neutralize bacterial toxins and viruses, and function as opsonins, stimulating phagocytosis.
Immunoglobulins are formed by light and heavy (depending on molecular weight) chains of polypeptides made up of about 100 amino acids. These chains determine the structure of antigen-binding sites and, therefore, the specificity of the antibody to one antigen. The five types of immunoglobulins (IgA, IgD, IgE, IgG, IgM) account for approximately 30% of all plasma proteins. Antibodies are one of the three classes of globulins (plasma proteins) in the blood that contribute to maintaining colloidal oncotic pressure.