||It has been predicted that depression will become the second-leading cause of disability worldwide after human immunodeficiency virus (HIV) in 2030. Numerous recent data suggest an important role of inflammation in patients with depressive disorder. In fact, there has been found an increase of inflammatory markers in some depressed patients and the administration of pro-inflammatory cytokines or their inducers has been shown to cause symptoms of depression. Besides this, it has been discovered that patients with chronic inflammatory diseases (rheumatoid arthritis, psoriasis, inflammatory bowel disease . . . ) show a higher prevalence of depression. Therefore, we hypothesize that manifestations of depression are related to chronic inflammation, at least in some patients with major depressive disorder. Thereby, an anti-inflammatory treatment may improve symptoms of depression. The general objective of this study is to analyse the relationship between inflammation and major depressive disorder (MDD) and search for potential diagnostic biomarkers and therapeutic targets.