IL6 and major depressive disorder: The persistent sympathetic and parasympathetic under-activity in chronic stress situations and major depressive disorder (MDD) increase catecholamine levels and decreases acetylcholine levels, which increases levels of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6, and IL-18, among others), which explains that depression-like symptoms can be induced directly by proinflammatory cytokines (Won et al., 2021).