Initially directed at the sole target of infectious disease, immunology has recently diversified its aim. Among its most important and promising new targets are malignant tumors. Given impetus by the observed proliferation of cancers in the immunosuppressed recipients of organ transplants—an observation that led to the inevitable conclusion that immune competence is essential to the control of malignant growths—immunologic techniques are being applied with increasing frequency to tumor diagnosis and treatment.
The principle underlying cancer immunology—and thus the basis of cancer immunotherapy—is the cell-mediated immune response of thymus-dependent (T) lymphocytes, as exemplified by the delayed hypersensitivity reaction to tuberculin. Patients to be treated are sensitized by topical application to normal skin of BCG vaccine or other nonspecific immunologic stimulant (the primary phase) and are later "challenged" (secondary phase) by administration of the antigen directly to the malignant lesion or its vicinity. The nonspecific immunologic stimulation potentiates the cytotoxicity of lymphocytes