immunoglobulin

immunoglobulin One of a group of proteins (globulins) in the body that act as antibodies. They are produced by specialized white blood cells called B cells and are present in blood serum and other body fluids. An antibody is typically a Y-shaped structure consisting of four polypeptide chains – two heavy chains and two light chains. Each arm of the ‘Y’ bears an antigen-binding site. There are five classes, with distinct immunological and physical properties. The main immunoglobulin of blood, lymph, and tissue fluid is immunoglobulin G (IgG). It binds to microorganisms, promoting their engulfment by phagocytes (macrophages), and to viruses and bacterial toxins, thereby neutralizing them. It also binds complement, which results in lysis of the target cell, and can cross the placenta to protect the fetus. Immunoglobulin M (IgM) is the first antibody to be produced following immunization or infection. Found in blood and lymph, it is an aggregate of five of the basic Y-shaped units, enabling it to ‘mop up’ microorganisms or other antigens with its ten binding sites. It can also activate complement. Immunoglobulin A (IgA) is found in saliva, tears, breast milk, and mucous secretions, where its role is to neutralize viruses and bacteria as they enter the body. It also occurs in serum. Immunoglobulin D (IgD) is present in serum in very low concentrations, but occurs on the surface of antibody-secreting B cells, whose activity it may regulate. It acts in conjunction with IgM as an antigen receptor. Immunoglobulin E (IgE) also normally has very low concentrations in blood and connective tissues, but it plays a crucial role in allergic reactions. It binds to mast cells, and when triggered by the presence of antigen causes histamine release and hence inflammation and other common allergic symptoms. See also immunity.