Anhedonic MDD showed increased serum levels of IL-2 (0.36 ± 0.14 pg/ml vs. 0.10 ± 0.01 pg/ml, t = 2.443, P = 0.021), IL-6 (4.12 ± 0.75 pg/ml vs. 1.46 ± 0.12 pg/ml, t = 3.493, P = 0.002), INF-γ (0.90 ± 0.43 pg/ml vs. 0.23 ± 0.11 pg/ml, t = 2.024, P = 0.046), and cortisol (17.28 ± 1.18 μg/dl vs. 10.95 ± 0.68 μg/dl, t = 4.891, P < 0.001) compared with the HC group (Figures 1G,I). Here, IL6 is linked to major depressive disorder.