TNF and major depressive disorder: More direct evidence of the relationship between inflammation and depression has been demonstrated by an increase in interleukin-6 (IL-6), C-reactive protein (CRP), and tumor necrosis factor-alpha (TNF-α) associated with a higher risk of depression and increased symptoms (Felger, 2018; Haapakoski et al., 2015; Huang et al., 2019; Khandaker et al., 2014; Paganin and Signorini, 2024; Sorri et al., 2017; Stephenson et al., 2024).