Studies suggest that endocrine and neuroendocrine systems greatly influence the immune system (
79). GH has an important role in the development of the immune system and may enhance the growth of the thymus gland which is responsible for the production of immune cells called T cells, the mediator of cell-mediated immunity (
80). GH is also produced by lymphoid organs, such as the thymus, the spleen, and immune cells (
81). Besides, clinical studies have suggested a significant role of GH in immune regulation, and the GH receptor is expressed on different subpopulations of lymphocytes (
82). GH stimulates T and B cell proliferation and immunoglobulin formation. It enhances the maturation of myeloid progenitor cells and is also able to modulate cytokine response (
83). A clinical study in 2012, demonstrated that lower circulating levels of IGF1 were associated with incidence and mortality from adult respiratory distress syndrome (ARDS). These data support the role of the IGF pathway in ARDS (
84). Having an immune-regulatory effect in addition to their anabolic effects, growth hormone (GH) and insulin-like growth factor 1 (IGF-1), may act to protect the host from lethal bacterial infection as well. The hormones promote the maturation of myeloid cells, stimulate phagocyte migration, prime phagocytes for the production of superoxide anions and cytokines and enhance the opsonic activity (
85).
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