IL18 and major depressive disorder: Beyond variations of the traditional pro-inflammatory cytokines IL-6, IL-1β, and TNF-α, depression has been associated with elevations of circulating IL-18, which is involved in cell-mediated immunity (Merendino et al., 2002; Prossin et al., 2011), and macrophage migration inhibitory factor (MIF), which is considered to be a pro-inflammatory cytokine that has neurogenic actions (Musil et al., 2011; Cattaneo et al., 2013).