Psychoneuroimmunology (PNI) is a discipline that has evolved in the last 40 years to study the relationship between immunity, the endocrine system, and the central and peripheral nervous systems. In this manner, neurotransmitters, hormones, and neuropeptides have been found to regulate immune cells, and these in turn are capable of communicating with nervous tissue through the secretion of a wide variety of cytokines. Of critical importance is the effect of products of the CNS and nerves on the maintenance of the delicate balance between cell-mediated (Th1) and humoral (Th2) immune responses. A good example of how this concept operates in vivo becomes evident when analyzing the effects of stressors. Chronic stress affects significantly the function of the immune system as well as modifies the evolution of a variety of skin diseases, as psychosocial interventions have proved to be effective in their therapy.