There have been many anecdotal reports that regular, moderate exercise confers some protective immunity against infection. There has been little scientific evidence to support this. It is also unclear whether training alters lymphocyte trafficking from the spleen to the periphery after a bout of exhaustive exercise.Methods and ResultsTo determine the effect of moderate training on in vivo antibody production, using rats as an animal model, we gradually trained 18 rats using a swimming protocol for a 4-week period after injection and booster with Keyhole limpet hemocyanin antigen. There were 9 age-matched controls. At the conclusion of training, both groups underwent a short-term exhaustive swim. The trained group showed marked enhancement of IgM and IgG production. After short-term exercise, both groups had acute lymphocytosis, mainly Tsuppressor/cytolyticand natural killer cells with decreases in Thelper(trained), B cells, and the Th-to-Ts, ratio. The changes in the splenocyte subsets were the opposite of the changes in the peripheral blood. With respect to function, after exhaustive exercise, there was a slight increase in mitogenesis and interleukin- 2 receptor expression to concanavalin A (untrained more than trained) compared with controls.ConclusionsRegular, moderate training enhances antibody production to specific de novo antigen both early and late. In addition, short-term exercise leads to selective release of immune cells from the spleen and results in slightly enhanced function of splenocytes. Direct stimulation by the sympathetic nervous system and catecholamines is the proposed mechanism for the changes seen after short-term exercise and possibly antibody production during training.