TNF and major depressive disorder: Moreover, an imbalance between pro-inflammatory cytokines (IL-1β, IL-6, IFN-γ, and TNF-α) and anti-inflammatory cytokines (IL-4 and IL-10) is frequently observed in untreated depressed patients [10,11], and increased levels of pro-inflammatory cytokines have been found to correlate well with severity of depression [12].