Natural killer cell cytotoxicity and T-cell proliferation is enhanced by avoidance behavior

Natural killer (NK) cell activity and mitogen-stimulated spleenocyte proliferation were measured in rats exposed to stress in the form of avoidable and unavoidable shock. Rats that could avoid shock exhibited higher NK activity than either unshocked controls or rats that could not avoid shock. The latter were yoked to the avoidance rats and thus received the same number and frequency of shocks as did the avoidance group. The increased NK activity in the avoidance group appears due to a higher number of NK cells in this group as compared with those in the control or unavoidable shock groups. Additionally, NK activity was found to be proportional to avoidance response rate, with a majority of animals exceeding the minimal temporal avoidance requirement. Mitogen-stimulated proliferation of spleenocytes was also increased several fold in the group that could avoid shock as compared with that which could not and controls. The difference in NK activity and mitogen-stimulated proliferation could not be ascribed to differences in cortisol levels. The results indicate that behavior which results in the avoidance of aversive stimuli can lead to significant enhancement of immune system competence.