In addition to these three pro-inflammatory cytokines, elevated levels of several anti-inflammatory cytokines [IL-2, IL-4, IL-10, transforming growth factor β (TGFβ)] have also been associated with MDD (Kubera et al., 2001b; Hernández et al., 2008; Sutcigil et al., 2008; Fazzino et al., 2009), whereas other cytokines, such as sIL-2R, CCL-2, IL-13, IL-18, and IL-12, have been found to be elevated in the blood of MDD patients in a few studies (Maes, 1995; Kim et al., 2002; Merendino et al., 2002; Pavón et al., 2006; Köhler et al., 2017). Here, IL18 is linked to major depressive disorder.