The cytokine-HPA axis feed-back circuit.

Due to their anti-inflammatory effects and their capacity to affect the synthesis of most cytokines and the Th1/Th2 balance, endogenous glucocorticoids contribute to control the basal operation of immune cells. Immune processes result in the production of cytokines, such as IL-1, IL-2, IL-6, IL-11, IL-12, TNF-alpha, and INF-gamma, which can activate the HPA axis. In turn, the resulting increase in glucocorticoid blood levels can affect immune cell activity directly or by controlling the production of cytokines, suggesting the existence of an immunoregulatory cytokine-HPA axis circuit. There is evidence that this circuit plays a relevant role in controlling excessive inflammatory reactions and the non-specific expansion of immune cells with no or low affinity for the antigen that triggers an immune response.