Concentrations of the pro-inflammatory cytokine IL-6 were highest in the Abuse+Depression group (1.10 log pg/mL), followed by Abuse (1.08 log pg/mL), Depression (0.97 log pg/mL) and Control (0.58 log pg/mL) (p = 0.05) (Fig 2C, Table 2B). Here, IL6 is linked to major depressive disorder.