TNF and major depressive disorder: Exposure to stressors such as immobilization stress (IS) and social defeat stimulates the secretion of adrenaline and glucocorticoids via the hypothalamic–pituitary–adrenal (HPA) axis, as well as tumor necrosis factor (TNF)-α and interleukin (IL)-6 in immune cells, leading to the onset of anxiety/depression and gut inflammation [8].