TNF and major depressive disorder: Some epidemiology and animal studies have shown that there is a close link between inflammation and depression, the levels of proinflammatory cytokines such as interleukin-1b (IL-1b), interleukin-2 (IL-2), interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-a) and anti-inflammatory cells such as interleukin-4 (IL-4), interleukin-10 (IL-10) are higher in patients with depression (12–16).