Over the past 20 years a variety of studies have demonstrated that exercise induces considerable changes in circulating leukocyte subpopulations and systemic levels of cytokines. The interactions between exercise stress and the immune system provide a unique opportunity to link basic and clinical physiology and to evaluate the role of immunophysiological mechanisms. It has been suggested that exercise represents a quantifiable model of physical stress as many clinical physical stressors (e.g., surgery, trauma, burn, sepsis) induce a pattern of hormonal and immunological responses that have similarities to that of exercise. Exercise-induced immune changes may be of importance for the individual's defense against infectious diseases. During the past years, there has been an increasing focus on the fact that exercise-induced immune changes may also be mediators ...