Systemic host defenses include phagocytic cells (neutrophils and macrophages), humoral immunity, and cell-mediated immunity. It may be beneficial to increase host defenses to prevent and treat infections, cancer, and often acquired or systemic immunodeficiency disorders. On the other hand, inhibition of immune responsiveness is desirable to prevent and treat rejection of organ grafts, allergic reactions, and autoimmune diseases. There are three general ways to modulate host defenses--pharmacologic agents, radiation, and biologic manipulation. The first two have been most widely applied but lack specificity. Thus, they generally inhibit a variety of immune responses that can lead to undesirable side effects such as infections and cancer. Nonspecificity is generally desirable in stimulating neutrophils and macrophages because it may not be known which bacterium is causing the infection or because several may be involved. Biologic manipulations have the advantage of great specificity but are much more difficult to apply to humans. Some of these types of manipulations that seem to hold promise are stimulators of phagocytosis, immunologic tolerance, control of specific subsets of T lymphocytes, monoclonal antibodies, and interferon. Immune modulation has a price-cancer, susceptibility to infection, toxicity of drugs, serum sickness, anaphylactic reactions, Arthus reaction, and disseminated infection with live viral vaccines.