Many athletes believe that habitual exercise protects them against infection. This article reviews ten studies of the effects of exercise on various host-defense factors. Exercise produced a transient granulocytosis and lymphocytosis, and in some studies, lymphocyte function was reported to have been enhanced. Serum immunoglobulin and complement levels were not significantly altered in the small number of subjects studied. Two recent studies showed that exercise produced an increase in circulating endogenous pyrogen in man. Since it now appears that endogenous pyrogen is identical to interleukin-1, a product of mononuclear cells that enhances lymphocyte function, it may play a role in host defense. Further studies will be needed before it can be concluded that exercise effects the host response to infection in any clinically meaningful way.
