In particular, it has been suggested that IL-6 is involved in multiple physiological systems, including the hypothalamic-pituitary-adrenal axis, corticotrophin-releasing hormone activity at limbic sites, noradrenaline utilization, the induction of oxidative stress, apoptotic pathways, and kinase signaling31–34, all of which have very close relationships with the pathophysiology of MDD. Here, IL6 is linked to major depressive disorder.