Fig. 2.

The endocrine system in brain–lung axis. The CRH is release from the hypothalamus onto the pituitary gland to allow ACTH to enter systemic circulation. Circulation ACTH acts quickly and briefly to stimulate the adrenal cortex to synthesize and secrete corticosteroids such as GC. GC plays an immunosuppression role in immune cells, induces the apoptosis of epithelial cells and reduces the ability of mucociliary clearance. Interestingly, lung cancer, for instance, SCLC, could secret ACTH which negatively regulates the HPA axis. Besides, the increase of catecholamine is induced by the activation of SAS after stress. NE increases the mobility and proliferation of bacteria, elevates the expression of bacterial toxins, and enhances the pathogenicity of bacteria. Furthermore, NE inhibits the maturation and activation of Th1 cells, and suppresses immune response, which ultimately increases the risk of infection