Experiments performed on CBA mice thymectomized in adult life, exposed to lethal doses of irradiation and given tissue therapy are described. Marrow, foetal liver, or spleen cells from syngeneic donors could protect the mice against the lethal effects of irradiation. Between 30 and 70 days' postirradiation, however, marrow-treated, thymectomized irradiated mice showed evidence of trophic disturbances, such as failure to gain weight, in contrast to sham-operated, irradiated, marrow-treated controls. The immune responses of experimental and control mice were tested up to 150 days' postirradiation by challenging with sheep erythrocytes and allogeneic skin grafts. Sham-operated irradiated controls, whether protected with marrow, foetal liver, or spleen cells, produced normal immune responses when challenged at 28, 60, or 150 days after irradiation. Neither foetal liver cells nor marrow cells, in doses of up to 40 million cells per mouse, enabled thymectomized irradiated mice to recover normal immune functions. Spleen cells, from normal donors but not from neonatally thymectomized donors, restored immunological capacity in such mice. It is concluded that immunologically competent cells are present in the spleen of normal adult donors and can function in the absence of the thymus. Bone marrow, on the other hand, does not contain an adequate population of such cells but has lymphoid precursor cells, the descendants of which can become immunologically competent only in the presence of a functioning thymus mechanism.

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