The neuroimmune hypothesis posits that inflammation may have a role in MDD as evidenced by elevated levels of markers of inflammation and oxidative stress, including interleukin-6 (IL-6), interleukin-17 (IL-17), interleukin-21 (IL-21), interleukin-35 (IL-35), tumor necrosis factor-alpha (TNF-α), 8-hydroxy-2′-deoxyguanosine (8-OHdG), and F2-isoprostanes in individuals with MDD [40,41]. Here, IL21 is linked to major depressive disorder.