IFNG and major depressive disorder: Unipolar depression (Major Depressive Disorder—MDD) has generally been found to stimulate a pro-inflammatory Th1 response (IL-1, IL-2, soluble IL-2 receptor (-sIL-2R), IL-6, C-Reactive Protein (CRP), TNF-α, and IFN-γ), with a subsequent increase in tryptophan catabolites (the kynurenine pathway) [17,22,23].