The evidence available is in areas of genetic links, association of depression with raised inflammatory markers such as Tumour Necrosis Factor (TNF)-alpha, Interleukin (IL)-1, IL-6, co-morbidity of depression with inflammatory medical illnesses, administration of cytokines leading to depression, and the recognition that anti-depressants have anti-inflammatory and neuro-protective properties. Here, TNF is linked to depressive disorder.