The immune system is designed to discriminate self from nonself. This goal is accomplished by recognition, effector, and memory mechanisms. At the cellular level the immune system consists of macrophages and lymphocytes. The former constitute a major line of phagocytic cellular defenses. Two classes of lymphocytes are recognized: B-lymphocytes and their differentiated cell types (plasma cells). Both secrete immunoglobulins. T-lymphocytes and their progeny derived from the thymus are the effector cells responsible for delayed-type hypersensitivity reactions. In addition, subsets of T-cells regulate and control the immune response by helper or suppressor activities. Interaction among these cell types within the host determines the outcome of many infectious diseases.