During the last 95 years, 629 papers (60 % in the 1990s) dealing specifically with exercise and immunology have been published. Major findings of practical importance in terms of public health and athletic endeavor include: (a) In response to acute exercise (the most frequently studied area of exercise immunology), a rapid interchange of immune cells between peripheral lymphoid tissues and the circulation occurs. The response depends on many factors, including the intensity, duration, and mode of exercise, concentrations of hormones and cytokines, change in body temperature, blood flow, hydration status, and body position. Of all immune cells, natural killer (NK) cells, neutrophils, and macrophages (of the innate immune system) appear to be most responsive to the effects of acute exercise, both in terms of numbers and function. In general, acute exercise bouts of moderate duration (< 60 min) and intensity (< 60 % VO2max) are associated with fewer perturbations and less stress to the immune system than are prolonged, high-intensity sessions. (b) In response to long-term exercise training, the only finding to date reported with some congruity between investigators is a significant elevation in NK cell activity. Changes in the function of neutrophils, macrophages, and T and B cells in response to training have been reported inconsistently, but there is some indication that neutrophil function is suppressed during periods of heavy training. (c) Limited data suggest that unusually heavy acute or chronic exercise may increase the risk of upper respiratory tract infection (URTI), while regular moderate physical activity may reduce URTI symptomatology. (d) Work performance tends to diminish with most systemic infections, and clinical case studies and animal data suggest that infection severity, relapse, and myocarditis may result when patients exercise vigorously. (e) Although regular exercise has many benefits for HIV-infected individuals, helper T cell counts and other immune measures are not enhanced significantly. (f) Data suggest that the incidence and mortality rates for certain types of cancer are lower among active subjects. The role of the immune system may be limited, however, depending on the sensitivity of the specific tumor to cytolysis, the stage of cancer, the type of exercise program, and many other complex factors. (g) As individuals age, they experience a decline in most cell- mediated and humoral immune responses. Two human studies suggest that immune function is superior in highly conditioned versus sedentary elderly subjects. (h) Mental stress, undernourishment, quick weight loss, and improper hygiene have each been associated with impaired immunity. Athletes who are undergoing heavy training regimens should realize that each of these factors has the potential to compound the effect that exercise stress is having on their immune systems.
