Rare individuals, experiments of nature who suffer the often devastating consequences of a congenital defect within the immune system, have served to teach us much of what we know today about the complex immune response of man (36,39,54). From a careful study of them and their diseases, together with related studies in the laboratory, we have learned, for example, that there are two major and quite separate arms making up the immune system of man and animals. These two basic components are now known as (1) cell-mediated immunity (CMI), or the T-cell (T for thymus derived) system, and (2) humoral immunity, or the B-cell (bone marrow or bursa-derived) system.