Immunoglobulins are members of that family of homologous proteins which includes the humoral antibodies. An antibody is a protein which appears in increased amounts in the body fluids of most vertebrates following the introduction of a foreign substance (antigen) into the body, and which is able to bind specifically to that antigen. In general, any protein which has not been in contact with the animal’s immune system during embryogenesis can act as an antigen, as will certain complex carbohydrates, lipids, or nucleic acids. Since each of these many different antigens appears able to elicit a different set of specific antibodies, the number of structurally different immunoglobulin molecules an animal is able to synthesize must be extremely large. The number of different antibodies a single animal can potentially make has not been experimentally determined, but estimates have ranged from several thousand to many million.