the general population with preliminary practical guidelines in the areas of exercise prescription, respiratory infection, aging, and athletic endeavor.

From early in this century, it has been regularly reported that during recovery from high-intensity, cardiorespiratory exercise, subjects experience a sustained neutrophilia and lymphocytopenia (Garrey and Bryan, 1935). Of all immune cells, natural killer (NK) cells, neutrophils, and macrophages (of the innate immune system²) appear to be most responsive to the effects of acute exercise, both in terms of numbers and function (Gabriel et al., 1992; Nieman and Nehlsen-Cannarella, 1994; Pyne, 1994). The longer and more intense the exercise bout (e.g., marathon race competition), the greater and more prolonged the response, with moderate exercise bouts (>60% maximal aerobic power and >60 minutes duration) evoking little change from resting levels (Nieman et al., 1989, 1991, 1993b, 1994).

Many mechanisms appear to be involved in the acute immune response to exercise, including exercise-induced changes in stress hormone and cytokine concentrations, body temperature changes, increases in blood flow, and dehydration (Brenner et al., 1995; Cupps and Fauci, 1982; Pedersen and Ullum, 1994).

Following prolonged running at high intensity, serum cortisol concentrations are significantly elevated above control levels for several hours (Nieman et al., 1995a) (Figure 17-1). Cortisol has been related to many of the immunosuppressive changes experienced during recovery (Cupps and Fauci, 1982). Glucocorticoids administered in vivo have been reported to cause neutrophilia, eosinopenia, lymphocytopenia, and a suppression of both NK and T-cell function, all of which occur during recovery from prolonged, high-intensity, cardiorespiratory exercise. Figure 17-2 demonstrates that a significant correlation exists between the change in serum cortisol and the change in the neutrophil/lymphocyte ratio following 2.5 to 3 hours of running (Nieman et al., 1995d). The neutrophil/lymphocyte ratio, which rises strongly after heavy, prolonged exertion, has been proposed as an excellent index of the physiologic stress on the immune system (Linden et al., 1991).