ROLE OF THE THYMUS IN HUMORAL IMMUNITY: Observations in Complete or Partial Congenital Absence of the Thymus

Abstract

Congenital absence of the thymus in man has been associated with markedly defective humoral immune responses in spite of normal immunoglobulin production. This is probably due to lack of thymus-dependent antigen-reactive cells. Some infants with incomplete defects in embryogenesis of the third and fourth pharyngeal pouches have very small thymus glands which are histologically normal but inadequate for optimal immunological function. Such individuals appear to have a significant but very limited pool of competent antigen-reactive cells. This situation seems to be almost identical to that found after neonatal thymectomy in rodents and fowl, or after reconstitution of irradiated thymectomised animals with bone-marrow. The nearly normal antibody responses to some antigens in such animals and in infants with partial defects is not considered to be evidence for a separate class of antigen-reactive cells independent of thymic function. Available data support the contention that at least a few thymus-dependent antigen-reactive lymphocytes are required for initiating production of any specific antibody. When thymic function is greatly reduced, however, it is proposed that the humoral immune response is limited primarily by the availability of competent thymus-independent (”bursal equivalent” precursors of antibody-forming cells. Thus the ”two-component” concept of immunological development is regarded as reconcilable with evidence that thymus-dependent antigen-reactive cells are needed to initiate both humoral and cellular immune processes.