Principles and Methods of Toxicology, 4th Edition. AW Hayes, ed., Philadelphia, Pennsylvania: Taylor & Francis, 2001 Jan; :1415-1450
The immune system is a complex, multi-cellular organ system consisting of granulocytes, macrophages, lymphocytes, and dentritic cells with various functions and phenotypic characteristics, as well as various soluble mediators. These cells of hemopoietic origin 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, and maturation. It exists to defend the body against invasion by infectious and opportunistic microorganisms, and spontaneously arising neoplasms. 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 (81).