Whereas the transfer of lymphocytes from normal syngeneic donors fails to abrogate tolerance of sheep erythrocytes in rats, lymphocytes from allogeneic donors are effective. When tolerance is abrogated in this situation, the hemolysin-forming cells are predominantly of host origin. Immunological interaction between transfused lymphocytes and host cells is a prerequisite for the abrogation of tolerance. From the time required for abrogation to occur after transfer of the allogeneic cells, it is suggested that tolerance of sheep erythrocytes in rats represents the repression of a specific reactivity in cells rather than the elimination or irreversible inactivation of reactive cells. This explanation implies the existence of specifically tolerant cells.

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