Exercise affects human immune functions through the hypothalamus-pituitary-adrenal (HPA) axis and reward systems in the brain. Most immune cells, including T cells, B cells and phagocytes, express functional hormone and neurotransmitter receptors and are regulated through the neuroendocrine immune network. Light-to-moderate exercise improves the defense activity against pathogens and prevents metabolic disorders. Many epidemiological and experimental studies have suggested there is an inverse relationship between moderate exercise training and illness risk. Furthermore, habitual exercise might improve immune regulation, reduce the activity of autoimmune disorders and delay age-related dysfunctions, such as sarcopenia. On the other hand, high-intensity exercise suppresses immune defense activity, especially against upper respiratory infections. Thus, high-intensity training and competitions possibly increase the risk of infection in athletes. Appropriate nutritional intake and hygiene control are also essential to protect both athletes and non-athletes.
