The immune system in its four components T cells, B cells, complement and phagocytes is vital for the defense of the organism against foreign substances or antigens. Elimination of an antigen involves the interaction of the four complements of the immune system with antigen and with each other. The final common pathway is the breakdown of the antigen by the intracellular machinery of the phagocytic cell. The maintenance of the self depends on the integrity of the immune system. Whenever this integrity is breached (by aging, congenital immune deficiency, cytotoxic drugs, irradiation, infiltrative disease...) the organism becomes prone to uncontrolled invasion by antigens (viruses, bacteria, fungi, neoplasms) and its survival is threatened.