We review here the evidence reminding us that the immune system and the brain have a close relationship that until recently has remained hidden. We begin with some of the facts concerning brain immune system interactions looking at the anatomical barriers and the mediators of immune and nervous system functions. We analyze the immunological events as well as some of the poorly understood symptoms of a devastating autoimmune disease of the brain such as multiple sclerosis. Finally, we consider

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... e clinical enigmas where there is evidence that in stress situations the brain can influence the immune system through modification of the hypothalamic-pituitary-adrenal axis (HPA), a key player in stress responses.