The immune system plays an important role in depression; the cytokine and neuroinflammation hypotheses suggest an increase in pro-inflammatory cytokines (interleukin 6 [IL-6] and tumor necrosis factor alpha [TNF-α]), and a decrease in anti-inflammatory cytokines (interleukin 10 [IL-10] and transforming growth factor beta [TGF-β]), contribute to a pro-inflammatory state in depressed patients. This evidence concerns the gene TNF and major depressive disorder.