A meta-analysis about MDD published in 2012, which included 29 studies of the proinflammatory cytokines in the serum of 1548 patients––822 MDD, 726 healthy controls—confirmed that soluble IL-2 receptor, IL-6, and TNF-α levels are increased in MDD (trait markers), while, IL-1β, IL-2, IL-4, IL-8, and IL-10, are not statistically different from controls (63). Here, IL6 is linked to major depressive disorder.