The proliferative response of human lymphocytes to antigen is suppressed preferentially by lymphocytes precultured with the same antigen.

Human blood lymphocytes activated in vitro with antigen to which the donor is reactive are capable of suppressing the secondary proliferative response of autochthonous fresh cells to antigen. Both antigen-specific and antigen-nonspecific suppression can be detected in each experiment. These suppressor cells act by decreasing the number of lymphocytes entering the proliferative response rather than by slowing or otherwise inhibiting ongoing proliferation. The suppressor cells must be added soon after fresh cells are stimulated with antigen to be effective, but the suppressor cells themselves need not proliferate to exert their effect. Suppressor cells are optimally effective when added in numbers equal to those of the responding population, but still exert a significant effect at one-eighth that number.