TNF and major depressive disorder: Numerous studies have found that MDD patients have increased serum concentrations of pro-inflammatory cytokines, such as interleukin-1β (IL-1β), IL-6, and tumor necrosis factor-α (TNFα) (Dowlati et al., 2010; Köhler et al., 2017; Kwon et al., 2017; Çakici et al., 2020; Reisinger et al., 2021).