A parallel meta-analysis revealed that several pro-inflammatory cytokines, including IL-1, IL-6, and TNF-α, were related to the development of depression (Coplan et al., 2014), and increased levels of pro-inflammatory cytokines, such as IL-6, C-reactive protein, and TNF-α, were also detected in the cerebrospinal fluid and serum of depressed individuals, who were otherwise healthy. Here, TNF is linked to major depressive disorder.