Health & Medicine


Antibody, any of perhaps a million kinds of normally occurring protein molecules that are produced in the body of cells called lymphocytes and that act primarily as a defense against invasion by foreign substances. An important component of the immune system, antibodies are found in the blood of all vertebrates, in the fraction of the blood called gamma globulin.

The synthesis, or manufacture, of antibodies is initiated when a foreign substance, referred to as an antigen, enters the body. Lymphocyte cells respond to the foreign substance by making an antibody with a molecular arrangement that fits the shape of molecules on the surface of the substance so that the antibody combines with it. Common antigens are the protein components of bacteria and viruses. These antigens may enter the body during infection or may be deliberately introduced by vaccination (see Immunization) in order to stimulate the production of antibodies. The binding of antibodies to the surfaces of bacteria, viruses, or toxins (see Toxin) can neutralize and eliminate these harmful substances in any or all of three ways: (1) by directly inactivating them, (2) by enabling other blood cells to engulf and destroy them (see Phagocytosis), and/or (3) by weakening their surfaces and rendering them vulnerable to destruction by other blood proteins (collectively called complement). Animals do not have antibodies to substances to which they have not been exposed, but one animal is able to produce enough different kinds of antibodies to fit the molecular arrangement of any foreign substance it is likely to encounter.

In diseases such as multiple sclerosis and systemic lupus erythematosus, the body mistakenly makes antibodies against normal tissue components (see Autoimmune Diseases). Sometimes viruses may disturb the immune mechanism.