Bidirectional interactions between the immune and neuroendocrine systems influence specifically physiological activities as diverse as tissue localization of lymphocytes, antibody responses, hypothalamic-pituitary hormone secretion, and neural signal transmission. Our understanding of intersystem communications has been increased by the delineation of the innervation of immune organs, effects of neuromediators on immune cells, and neuroendocrine responses to individual immune cytokines. Two patterns of responses to neuroimmune mediators have been defined to distinguish between direct alterations in cellular function and the more complex states of differentiation and adaptation that condition the threshold and nature of reactions to subsequent stimuli. Recent identification of the molecular mechanisms of action of neuroimmune mediators is exemplified by elucidation of the requirement for prolactin in T lymphocyte proliferation and of the effects of vasoactive intestinal peptide on B lymphocyte expression of adherence proteins. Further knowledge of the advantages of multisystem integration of functions in host defense may reveal other novel mechanisms of cellular communication and biological adaptation.
