Nervous system-immune system interactions: an overview.

Links between the nervous and immune systems are suggested by the behavioural conditioning of immunosuppression, the effects of brain lesions and stress on immune responses, and physiological and chemical changes in the brain during immune responses. These links probably include glucocorticoids secreted from the adrenal gland, catecholamines and neuropeptides secreted by sympathetic terminals and the adrenal medulla, certain pituitary hormones, and polypeptides produced by cells of the immune system. The effect of glucocorticoids is not exclusively immunosuppressive, nor is it adequate to explain all the effects of stress. In vitro endogenous opiates facilitate lymphocyte proliferation and natural killer (NK) cell activity, but in vivo opiates appear to inhibit immune responses and impair tumour rejection. Increases of circulating glucocorticoids after infection and an apparent activation of cerebral catecholaminergic cells indicate that challenges to the immune system are interpreted physiologically as stressors. Moreover, they suggest that the brain may be able to monitor the progress of immune responses. Certain protein factors produced by the thymus gland (thymosins) may be able to counter stress-induced deficits in immunological responses.