Neuroendocrine-immune interactions.

The role of the neuroendocrine system in influencing both immune development and function has become an area of active research within many model systems, including the chicken. It is now clear that the neuroendocrine system can exert immediate feedback regulation on the immune system as well as control specific aspects of immune differentiation and development. The primary lymphoid organs of avian species (i.e., the thymus and the bursa of Fabricius) are also known to function as endocrine organs. These produce hormonal products that influence the development of lymphoid cells and that may feed back on the neuroendocrine system. In conjunction with the endocrine activities of the primary lymphoid organs, immune and accessory cells are known to produce a variety of secreted products or cytokines that have the potential not only for the regulation of immune function but also for mediating neuroendocrine activities. Finally, it has been demonstrated in a variety of species that leukocytes are capable of producing endocrine mediators previously believed to be produced only under the direct control of the hypothalamic-pituitary axis. Thus, there are numerous possibilities for bidirectional interactions between the immune and neuroendocrine systems. This discussion focuses primarily on these interactions with an emphasis on the means by which the hormonal mediators, growth hormone and thyroid hormone, may affect the thymus and the thymic microenvironment. The role of the adrenocorticoids and gonadal steroids in regulating immune function and their involvement in immune feedback circuits are also discussed.