Antibodies are produced primarily by plasma cells and the precursor B lymphocytes in response to the presence of foreign molecules (antigens) in the body.
The antibody response by the lymphocytes does not occur in all living organisms and is known only to vertebrate recognition systems. The circulating antibodies bind to the specific antigen against which they are produced, and the resulting antigen-antibody complexes are removed from circulation by macrophages through phagocytosis. Antibodies are widely used in all areas of biological research, including clinical diagnostics, because of their specific interactions with antigens. Since antigens can specifically detect molecules in complex mixtures and in tissues, varieties of immunochemical techniques have been manipulated for investigation.