BDNF and major depressive disorder: Through responsing to chronic stress, the development of MDD has been facilitated and then the stimulation of the immune system is enhanced, which leads to an increase in the level of inflammatory mediators, such as IL-1β, IL-6, BDNF and TNF-α (tumor necrosis factor), and results in the activation of microglia. Then the persistence of an inflammatory response leads to impaired cognitive function