IL18 and major depressive disorder: Regarding the main cytokines examined in affective disorders, meta-analytic research has revealed that patients with depression have elevated serum or plasma concentrations of IL-6, IL-10, IL-12, IL-13, IL-18, TNF-α, and its receptor soluble TNFR2 compared to healthy controls, whereas IFN-γ levels are lower in patients with depression than healthy controls (2, 33, 61–63).