IFNG and major depressive disorder: Notably, in vitro studies have reported that high levels of anti-inflammatory cytokines, such as IL-4, are associated with elevated cortisol and can impede the capture of 5-HT [32], significantly downregulating cytokines, such as IL-2 and IFN-γ, in MDD patients [21], acting primarily through immunological antagonism between pro- and anti-inflammatory cytokines.