During exercise, body temperature rises as a result of increased energy metabolism and heat absorbed from the environment. In response to this rise in body temperature, blood flow increases and stress hormones are released. Together, blood flow and stress hormones stimulate increases in the number of circulating leukocytes and alterations in various aspects of immune function, including cytokine production. The extent of changes in leukocyte numbers, cytokine concentrations, and immune cell function depends on how high body temperature rises and the intensity and duration of exercise. In general, increases in body temperature of ≤1.8°F (1° C) induce mild changes in immune function, and such changes are unlikely to increase the risk of illness in athletes, firefighters, and military personnel who regularly exercise in hot conditions. More severe immune disturbances during exercise in extreme heat (≥106°F or 41° C) may contribute to classical symptoms of heatstroke.