immunoglobulin

any of a group of high molecular weight PROTEINS that act as ANTIBODIES and are present in SERUM and secretions. Designated Ig, there are five groups, each with different functions identified by a particular letter. Immunoglobulin A (Ig A) is the most common and occurs in all secretions of the body. It is the main antibody in the MUCOUS MEMBRANE of the INTESTINES , BRONCHI , saliva and tears. It defends the body against microorganisms by combining with a PROTEIN in the MUCOSA . Ig D is found in the SERUM in small amounts but increases during allergic reaction. Ig E is found primarily in the LUNGS , SKIN and mucous membrane cells and is an anaphylactic antibody (see ANAPHYLAXIS ). Ig G is synthesized to combat BACTERIA and VIRUSES in the body. Ig M, or macroglobulin, has a very high molecular weight (about five or six times that of the others) and is the first produced by the body when ANTIGENS occur. It is also the main antibody in BLOOD GROUP incompatibilities.