BDNF and major depressive disorder: Tumor necrosis factor-alpha (TNF-a), brain-derived neurotrophic factor (BDNF), C-reactive protein (CRP), and various interleukins (ILs)—including IL-5, IL-6, IL-8, and IL-10—have been implicated in the pathophysiology of MDD [5, 8, 11–15].