There is now evidence that depression is accompanied by activation of the immune-inflammatory response system (IRS) marked by increased levels of pro-inflammatory cytokines, such as interleukin (IL)-1, IL-6, tumor necrosis factor (TNF)-α, and interferon (IFN)-γ in the blood (Maes et al. 1991, 2009; Kubera et al. 2001, 2011b; Faugere et al. 2018; Dubois et al. 2018). This evidence concerns the gene IL6 and major depressive disorder.