IL6 and major depressive disorder: Studies have shown that the peripheral blood of first-episode MDD patients was in an inflammatory state, such as the increase of interleukin-1β (IL-1β), interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ) and the decrease of interleukin-2 (IL-2) (9).