Examples include (a) treatment of hepatitis C with IFN-α results in the onset of major depressive disorder (MDD) in around 50 % of patients and is the most common reason for ceasing treatment [3–5], (b) the treatment of psoriasis patients with the anti-TNF drug etanercept improved the clinical symptoms of depression independently of an improvement in psoriatic disease score in patients [6, 7] and (c) patients suffering from long-term MDD or schizophrenia have an elevated inflammatory profile [8, 9]. Here, TNF is linked to major depressive disorder.