The established mechanisms of immune responsiveness to foreign or self components are reviewed, with particular reference to relevant clinical problems and current research. A multitiered immunological system of cellular and subcellular elements are involved when the body deals with perturbations from without or within. The concept exists that a delicate balance between positive ('helper') and negative ('suppressor') forces is essential to maintaining health. Brief discussion is given to diagnosis of immune abnormalities in the light of these facts.