IL6 and major depressive disorder: The authors summarized that although most studies showed a link between abnormally higher IL-1β, IL-6, TNF-α, transforming growth factor (TGF)-β1, vascular endothelial growth factor (VEGF), kynurenic acid (KYN), and lower IL-2, IL-4, and interferon (IFN)-γ levels in specific brain regions and suicidal behavior, the contribution of MDD as a mediator of the link between these cytokine abnormalities and suicidal behavior could not be excluded.