BDNF and major depressive disorder: In this study, inflammatory factors, neurotrophic factors, and inflammatory proteins, such as TNF-α, IL-4, IL-6, IL-10, IL-18, IL-23, proBDNF, BDNF, hs-CRP, and HMGB1, were chosen as targets for investigation based on the association between immune inflammation, the nervous system, and MDD in this study (19).