IL17A and major depressive disorder: Elevated levels of pro-inflammatory cytokines such as IL-17, IL-21 and IL-23, and reduced levels of IL-35, have been frequently observed in patients with MDD compared with healthy controls, suggesting an enhanced pro-inflammatory gene expression.63 Emphasising the role of cytokines and the immune response in the pathogenesis of depression, another study demonstrated a significant increase in serum levels of IL-12 and IL-4 in patients with MDD, with a positive correlation between these cytokine levels and the severity of depression.84