Elevated blood IL-17 levels are observed in patients with depression,86–88 and chronic stress-induced IL-17 contributes to depression-like behaviors through neuroinflammation in mice.89 Conversely, IL-17 deficiency has been associated with social avoidance behaviors,90,91 suggesting that an appropriately balanced IL-17 response is crucial for normal neuroimmune function. This evidence concerns the gene IL17A and major depressive disorder.