A meta-analysis of 12 studies examining the association between OCD and circulating cytokines (tumor necrosis factor (TNF-α), interleukin (IL)-1β, IL-6) revealed the following: (i) reduced levels of IL-1β in OCD compared to healthy controls, (ii) elevated IL-6 levels in treatment-naïve OCD patients compared to those who had received treatment, and (iii) increased TNF-α levels in OCD patients with comorbid depression [97]. Here, IL6 is linked to depressive disorder.