Modification of the tolerant state by neonatal thymectomy.

Immunological tolerance of sheep erythrocytes was induced by repeated antigenic challenge of rats which had been thymectomized on the day of birth. Thoracic duct lymphocytes from these thymectomized tolerant rats failed to mount adoptive responses against sheep erythrocytes and interfered with the capacity of normal lymphocytes to mount such responses. However, in sharp contrast with non-thymectomized tolerant rats, thymectomized animals responded well to sheep erythrocytes if challenge was accompanied by the injection of normal syngeneic lymphocytes. It is likely that the cells that are lacking from tolerant rats as a consequence of neonatal thymectomy are identical with the radiosensitive cells that suppress normal lymphocytes transferred to intact tolerant rats.