Empirical data strongly supports the interaction between the nervous system and susceptibility to disease. During periods of psychological stress the incidence of both mild and severe disease increases significantly. While these findings provide evidence of the ability of the nervous system to influence immune function there is also a growing body of data that indicates that the immune system nay also effect neuroendocrine function.

Regulation of both the neuroendocrine and immune system is in large part via secondary messengers (hormones and lymphokines) whose production and secretion mediate activity of specific cells. The functional interactions between the neuroendocrine and immune system is evidenced by the ability of several neuroendocrine hormones to influence (both positively and negatively) immunologic functions and by the production of neuroendocrine hormones by stimulated lymphocytes. In many cases, this "cross-talk" appears to be the consequence of the production of secondary messengers common to both the immune and neuroendocrine systems.

As our understanding of the interrelationships between the immune and neuroendocrine systems increases, a more accurate picture of the interactions between these systems will merge. The implications and potential usefulness of this information will be to allow a determination of when physiologic or environmental stressors detrimentally alter the defense systems that are responsible for providing protection from disease.