antibody

antibody protein produced by the immune system (see immunity ) in response to the presence in the body of antigens: foreign proteins or polysaccharides such as bacteria, bacterial toxins , viruses, or other cells or proteins. Such antigens are capable of inflicting damage by chemically combining with natural substances in the body and disrupting the body's processes. The body contains hundreds of thousands of different white blood cells called B lymphocytes, each capable of producing one type of antibody and each bearing sites on its membrane that will bind with a specific antigen. When such a binding occurs, it triggers the B lymphocyte to reproduce itself, forming a clone that manufactures vast amounts of its antibody.

The antibody molecule is composed of four polypeptide chains (see peptide )—two identical light chains and two identical heavy chains—joined by disulfide bridges. The light chains have a variable portion that is different in each type of antibody and is the active portion of the molecule that binds with the specific antigen. Antibodies combine with some antigens, such as bacterial toxins, and neutralize their effect; they remove other substances from circulation in body fluids; they bind certain antigens together, a process known as agglutination; and they activate complement, blood serum proteins that cause the destruction of invading cells.

See also monoclonal antibody .

antibody A complex protein that is produced in response to the introduction of a specific antigen (which is normally foreign to it) into an animal. Antibodies are usually highly specific, combining only with antigens of a particular kind. Antibodies belong to a class of proteins called immunoglobins, which are formed by plasma cells in the blood as a defence mechanism against invasion by parasites, notably bacteria and viruses, either by killing them or rendering them harmless. The specificity of their binding reaction with a particular antigen is owing to the configuration of a particular small area, known as the active site, on the surface of the antigens. Thus when a parasite (or its poisonous products) enters the tissues of its host, the antigens deriving from the parasite each produce a particular response according to the specific antibody that binds to that antigen. This ‘recognition’ by the host of the species or strain of parasite which has entered is sometimes applicable to other parasites should they share the same antigen. For example, the vaccinia and smallpox viruses share the same antigen, so that immunity to one confers immunity to the other. Antibodies may persist in the body long after the disappearance of the antigen, so conferring immunity to any new infection by the same strain or species of parasite. Vaccination or inoculation provides immunity to an organism by the injection of particular foreign proteins (not necessarily from a parasite) which then stimulate the production of antibodies.

antibody (an-ti-bodi) n. a special kind of blood protein that is synthesized in lymphoid tissue in response to the presence of a particular antigen and circulates in the plasma to attack the antigen and render it harmless. Antibody formation is the basis of both immunity and allergy.