IL17A and depressive disorder: Although it has been clinically shown that the signature cytokine interleukin 17A (IL-17A) produced by Th17 cells does not significantly correlate with the severity of depression, Th17 cells are involved in the gut-brain axis to mediate the stress response, possibly by promoting neuroinflammation, microglia, and astrocyte activation thereby neuronal damage triggering depressive symptoms.