Patients with MDD had elevated serum levels of IL-2 (0.36 ± 0.07 pg/ml vs. 0.10 ± 0.01 pg/ml, t = 2.855, P = 0.005), IL-6 (2.62 ± 0.38 pg/ml vs. 1.46 ± 0.12 pg/ml, t = 2.922, P = 0.004), and cortisol (16.79 ± 0.76 μg/dl vs. 10.95 ± 0.68 μg/dl, t = 5.703, P < 0.001) compared with the HC group (Figures 1A,C). Here, IL6 is linked to major depressive disorder.