THE IMMUNE system is composed of a large and complex set of elements that are widely distributed. It is designed to protect against foreign pathogens while not responding adversely to self-components. Thus, it must distinguish "self" from "not self." This distinction is made by an elaborate, specific recognition system that, ultimately, is composed of receptors on T and B lymphocytes—the only immunologically specific components of the immune apparatus. The immune system also has nonspecific effector mechanisms that usually amplify the specific functions. These nonspecific features include mononuclear phagocytes, polymorphonuclear leukocytes, and the complement system. The immune system has several interesting characteristics. Specificity Immune reactions distinguish between non-cross-reacting antigens. Specificity is mediated through the antigen-specific receptors on the surfaces of T and B lymphocytes and through antibodies. Memory The immunologic response to an antigenic stimulus usually leaves the immune system changed. Ordinarily, a second administration of antigen results in an anamnestic,