THOUGHTS ON THE THYMUS AND THE T-LYMPHOCYTE REPERTOIRE - RELEVANCE TOTHE TOLERANCE OF THE IMMUNE-RESPONSE

The thymus is a central lymphoid organ, and has one important function in the immune system. It generates immunocompetent T (= thymus-depend ent) cells. 'Immunocompetence' is the single most important characteri stic acquired during the differentiation process of T-cells, from prog enitor cell in the bone marrow to mature cell in peripheral lymphoid o rgans and blood. The basis of competence is recognition, mediated by t he T-cell receptor on the cell surface. Using this receptor, T-cells r ecognize antigens only in the form of antigenic fragments complexed wi th molecules of the major histocompatibility complex. Within the thymu s, progenitor cells rearrange gene segments encoding the various parts of the receptor molecule. After expression of all possible receptors (the potentially available repertoire) selection processes occur, yiel ding only those T-cells that have a receptor by which antigens can be recognized together with molecules of the major histocompatibility com plex, and that do not recognize autoantigens (the actual repertoire). This process of selection is considered as a unique function of the th ymus. In this review, first some histophysiological aspects of the thy mus are described. Next, the events that occur during the sojourn of t he lymphocytes in the thymus are presented. Finally, these data are co nsidered with regard to thymic abnormalities. These include acute and age-associated involution, and the generation of autoimmune reactivity due to disturbed intrathymic selection.