In Major Depressive Disorder (MDD), neuroendocrine-immune interactions, tightly regulated by HPA axis under homeostatic conditions [236], cause imbalances in levels of neurotransmitters such as 5-HT and NE, hormones such as cortisol, and also in cytokines such as TNFα and IL-6 (variations of which show the closest association with depression), and also IL-1β, IL-4, IL-2, IL-8, IL-10 and IFN-γ, that contribute to the behavioral and immune disturbances observed in these patients [237-245]. This evidence concerns the gene IL4 and major depressive disorder.