There is now evidence that IL-6 is elevated in MDD patients, although antidepressants may increase (Maes et al. 1997; Kubera et al. 2000, 2004; Fornaro et al. 2011) or decrease (Xia et al. 1996; Himmerich et al. 2010; Köhler et al. 2018; Duda et al. 2019) serum IL-6 levels. Here, IL6 is linked to major depressive disorder.