ABSTRACT
The immune system is a complex multicellular organ system consisting of granulocytes, macrophages, lymphocytes, and dendritic cells with various functions and phenotypic characteristics, as well as various soluble mediators. These cells are of hemopoietic origin and, in adults, are found in the peripheral blood, lymphatic fluid, and organized lymphoid tissues, including bone marrow, spleen, thymus, lymph nodes, tonsils, and mucosa-associated lymphoid tissue. The immune system is in a constant state of self-renewal involving cell proliferation, differentiation, activation, and maturation. It exists to defend the body against invasion by infectious and opportunistic microorganisms and spontaneously arising neoplasia. This network of cells and soluble factors is highly regulated and interdependent, must discriminate self from non-self, and can react to non-self with many different (pleiotropic) defensive responses [1]. In addition, this immune system occasionally develops a response to a chemical or its metabolite that binds to or alters a host protein, resulting in allergy or autoimmune disease. The immune systems of experimental animals, although exhibiting some obvious differences from that of humans, are still sufficiently similar that data obtained from lower species are instructive of a potential human response [2].
