A study was made of the thymus’s role in the restoration of immune competence and the regeneration of the lymphoid system in X-irradi- ated mice. Thymectomy of adult mice followed by total body irradia¬tion at sublethal doses postponed or abolished the spontaneous reac¬tivation of the homograft reaction against genetically foreign tumors. Retransplantation of intact thymuses can reactivate the immune response. The depletion of the lymphoid system in spleen and lymph nodes of thymectomized and irradiated mice was studied. Following depletion of the lymphatic follicles, erythroblastic noduli proliferate in the spleen. In thymectomized animals, their appearance precedes that of irradiated controls. In spleens of the irradiated animals, erythro- poiesis predominated for about 4 weeks after which regression of of erythropoiesis took place with a concomitant regeneration of typi¬cal lymphoid follicles. In thymectomized animals, erythropoiesis per¬sisted for a much longer time (2 months and more), and following its eventual regression the regeneration of the lymphoid elements was either suppressed or abolished. Regrafting of thymuses to thymec¬tomized animals was followed by a lymphoid regeneration in the spleen. Tentative interpretation of the results of immune reaction following thymus grafts of isologous and homologous origins seem to support the concept that the thymus induces the differentiation to immune competent cells of lymphoblasts originated in the spleen or bone marrow, rather than being itself a tissue origin for immune competent cells.
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