ABSTRACT
Immunoglobulins first appear in the most primitive vertebrates. According to Paper-master et al. the Californian hagfish, a cyclostome, is unable to produce immunoglobulins and shows no immune response after antigenic challenge, although its phagocytes provide cellular defense. However, R. T. Acton et al. found bactericidal antibody response in the same species. The sole immunoglobulin of the fish and of the tadpole is the IgM. IgG may have appeared with the early terrestrial vertebrates, namely, in the lungfish. In the frog IgG appears in late metamorphosis, in association with the appearance of plasma cells in the lamina propria of the gut. In mammals, except in the most primitive species, the immune system of the young develops to a considerable extent in the protected environment of the uterus. In the fetus and newborn, suppressor lymphocytes dominate the lymphocytes in the immune system and an exposure to antigen tends to elicit tolerance instead of an immune response.
