ABSTRACT A bi-directional relationship between the neuro-endocrine system and immune function has long been postulated. Neurohormones act on immune cells while cytokines, secreted by lymphocytes and macrophages, in turn, influence the neuroendocrine system. The variability of endocrine and immune responses depends on these interactions which can be facilitatory or inhibitory. The presence of cell surface receptors for growth hormone (GH) and insulin-like growth factor-I (IGF-I) on different lymphocyte subpopulations suggests that, in addition to the endocrine effects, there is an immune action for these hormones. In vitro and animal studies have demonstrated an important role of GH on the immunoregulation. In fact, GH stimulates T and B cell proliferation and immunoglobulin synthesis, enhances the maturation of myeloid progenitor cells and modulates cytokine response. However, in humans, GH deficiency (GHD) is not usually associated with immunodeficiency and only minor abnormalities of immune function have been reported, when compared to those observed in GHD animals. It is possible that in humans GH produced locally in the immune system compensates for the lack of endocrine GH.
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