Exercise has been recognized as a stress, which can significantly alter the host's immune response and, therefore, its susceptibility to disease. Whereas research in this area has previously focused primarily on human subjects and laboratory animals, it has more recently extended to domestic animals, especially the equine athlete. Despite several studies, defining the relationship among exercise, the immune response, and disease has proven difficult due to a number of factors, including the complexity of the immune system and the variable nature of exercise itself. It now appears that exercise has dual effects on the immune system. Supn association between exercise and disease susceptibil-A ity was recognized as early as 1918, when it was reported that pneumonia was more common in human athletes than in sedentary individuals, and that strenuous exercise increased the risk of upper respiratory tract infections progressing to pneumonia.' Since that time, several studies have investigated the relationship among exercise, the immune response, and disease, leading to recognition that the stress of exercise has profound but variable effects on the immune Although comparatively little is known about the specific effects of exercise on immunity in domestic animals, the strenuous exercise that horses and other performance animals routinely engage in makes this subject of importance in veterinary medicine. In this article, we review the literature examining the relationship between exercise and immunity, and its clinical relevance.
Effects of Exercise on the Immune SystemConsiderable research has investigated the effects of exercise on the immune system, and while it has been established that virtually every aspect of both the nonspecific and specific immune systems can be affected by exercise, the definitive results of these studies often appear conflicting.*-"' Several factors contribute to the variability among studies, including the complexity of the immune system and differences in experimental design. Function of the immune system depends on a number of cell types, receptor systems, soluble mediators, and their interactions. The effects of exercise on isolated components of the immune system may not reflect a change in the overall immune status. Likewise, because variations in the intensity, duration, or specific type of activity can have significant effects on the response of the immune system, the nature of the exercise being studied
