IL6 and major depressive disorder: It has been shown that the levels of pro-inflammatory cytokines, mainly the interleukins (IL), such as, IL-1, IL-6, IFN-γ, and TNF-α, are elevated in the serum of patients suffering from depression (Dowlati et al., 2010; Schmidt et al., 2014; Haapakoski et al., 2015).