The mechanism of unresponsiveness in lymphocytes specifically selected by antigen in normal rats.

The immunological potentialities of populations of thoracic duct lymphocytes collected from rats recently challenged with sheep erythrocytes were compared with those of lymphocytes from rats immunologically tolerant of this antigen. Both "selected" and tolerant populations fail to mount immune responses to sheep erythrocytes in irradiated syngeneic hosts but can do so if transferred to semi-allogeneic recipients. If co-cultivated with normal lymphocytes in irradiated hosts, both types of population can modulate the responsiveness of the former to sheep erythrocytes. Supplementation of normal lymphocytes with "selected" populations collected earlier than 28 hours after challenge of the donors tends to result in augmented responses while later collections are more likely to be associated with suppression. Consequently, it is suggested that "selected" populations are unresponsive because of the presence of specifically activated cells rather than as a result of their depletion. The influence of "selected" populations on the responsiveness of normal lymphocytes is attributed to this content of activated cells.