IL6 and major depressive disorder: More recently, an abundance of observational, experimental and clinical evidence has emerged to suggest that the activation of innate immune mechanisms, especially proinflammatory cytokines IL-1, IL-6 and tumor necrosis factor alpha (TNF-α), as well as C-reactive protein (CRP), may contribute to the initiation and progression of psychiatric diseases, such as depression (Capuron et al., 2003, Dowlati et al., 2010, Gimeno et al., 2009, Howren et al., 2009, Kivimaki et al., 2014, Liu et al., 2012, Raison et al., 2010, Valkanova et al., 2013).