Research in psychoneuroimmunology has demonstrated that biopsychosocial factors such as psychological stress can influence the immune system. Chronic stress has been associated with the suppression of the immune function. In contrast, acute psychological stressors and physical exercise have been shown to transiently enhance immune responses. These stress effects on immunity seem to be mediated via endocrine factors, since hormones, neurotransmitters, and neuropeptides can interact with cellular components of the immune system. In summary, experimental and clinical evidence suggests a functional relationship between stress, immunity, and diseases.