IL6 and major depressive disorder: Meta-analysis of clinical studies in MDD has highlighted increased blood levels of various pro-inflammatory cytokines and their soluble receptors, such as interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-alpha), IL-1β, soluble IL-2 receptor, and C-reactive protein (CRP), in MDD compared to healthy controls (15–17).