TNF and major depressive disorder: Metabolites from the gut, microbial cell components, and even the gut microbiota itself translocate systemically through a compromised gut barrier, often referred to as “leaky gut.” This exacerbates systemic inflammation, such as Th17/Treg imbalance, IL-6, IL-1β, and TNF-α, all of which are closely related to the pathogenesis of depression.