Passage of Antigens across the Vascular Barrier of the Thymus

Abstract

ALTHOUGH it has been shown that the thymus has a fundamental role in the development of immunological response mechanisms^1–4, the formation of antibodies in the thymus itself under normal conditions has not been observed^5,6. Because antigen injected directly into the thymus does result in the formation of thymic germinal centres and the appearance of plasma cells⁷, it has been assumed that there is a blood–thymus barrier which inhibits the passage of antigens from the vascular system into the extravascular spaces of the thymus^6,7. In investigations with the electron microscope some evidence for the existence of such a barrier has been found^8,9. On the other hand, Clark has concluded that there is no absolute barrier to the penetration of antigen into the thymus although there may be a decreased permeability of the thymic parenchyma to antigen as compared with other lymphoid organs: iodinated human serum albumin, ferritin and trypan blue have been identified in the thymic parenchyma of mice injected with these substances¹⁰.