TNF and major depressive disorder: Chronic inflammation, a low-grade inflammation in depression,[51] may impair antiviral defenses by suppressing Th1-type immune responses through pro-inflammatory cytokines such as interleukin (IL)-6 and tumor necrosis factor (TNF)-α.[52] Th1 responses are critical for combating intracellular pathogens such as viruses and Mycobacterium tuberculosis.