The components and functions of the immune system are described, and the clinical applications of agents that affect the immune system are discussed. Through both nonspecific and specific responses, the immune system recognizes and destroys or eliminates harmful foreign substances with which a host comes into contact. Nonspecific responses are usually associated with the first introduction of a foreign substance into the body and consist mainly of phagocytosis and inflammation. When the same substance, or antigen, is introduced into the body on subsequent occasions, antibodies specific for that antigen combine with the antigen and activate a complex network of specialized cells and soluble cellular secretions that eliminate the substance from the body. Certain antigens are inherited and are found on the cells and tissues of the body; these antigens play a major role in human allotransplantation, blood transfusions, and certain disease states. Recent advances in biotechnology have made it possible to alter an individual's immunologic response with such agents as azathioprine, cyclosporine, or monoclonal antibodies or to augment an individual's antitumor defenses with immunopotentiators. As the products of biotechnology are used more frequently in the hospital setting to treat or prevent disease, pharmacists will need to have a good understanding of the immune system to appreciate the functional capacity of and the problems that may exist with these agents.