n. 1. (Med., Physiol. Chem.) Any of various bodies or substances in the blood which act in antagonism to harmful foreign bodies, as toxins or the bacteria producing the toxins. Normal blood serum apparently contains various antibodies, and the introduction of toxins or of foreign cells also stimulates production of their specific antibodies by the immune system. In certain “autoimmune” diseases, the immune system of an organism may produce antibodies to parts of the organism's own structure, and can cause considerable damage and even death as a result. [ Webster 1913 Suppl. +PJC ]

2. (Med., Physiol. Chem.) more narrowly, any of the immunoglobulins present in the blood serum or other body fluids of an animal, which reacts with a specific antigenic substance, whether the antibody was produced as a consequence of the stimulus provided by the antigen, or was pre-existing prior to exposure of the organism to the antigen. The soluble antibodies present in serum are complex globins (proteins) with both constant and variable regions in the polypeptide chains, the variable region being responsible for the reaction of the antibody with its specific antigen. Typically, antibodies of more than one structure may react with a given antigen, and any given antigen may stimulate production of more than one antibody. Methods have been developed to grow cells in tissue culture which produce predominantly only one specific antibody, and such antibodies thus produced (called monoclonal antibodies) are highly specific in their reactions to molecular strucures, and are used as valuable reagents in biochemical technology. [ PJC. ]