The fundamental ways in which the immune system has of recognizing and responding to the introduction of foreign molecules is identical, whether the source of the foreign molecules is a microbial infection, an allograft, an allergen, an autoantigen, or a tumor antigen. It follows that immune-based therapies that focus on promoting the quantity and quality of the immune response should be beneficial in the treatment of a range of diseases, especially persistent viral infections and cancer. Moreover, treatments that diminish the quantity and quality of the immune reaction should be beneficial in the treatment of allergies, allograft recipients and autoimmune disorders. Clinical trial design has been used successfully in persistent infections.