This is a study to characterise the role of inflammatory processes in depression. There is compelling evidence that inflammation is often associated with, and can cause, depression. It is currently less clear that antiinflammatory drugs have meaningful antidepressant effect. One of the goals is to identify the subset of depressed patients that is most likely to respond better to an antiinflammatory drug than to a conventional antidepressant. The investigators will therefore undertake a study of patients with a diagnosis of major depressive disorder including four groups: i) incompletely responsive patients who have demonstrated failure to respond consistently or completely to standard treatment, ii) those who have responded well to treatment and are not currently depressed, iii) untreated patients who are currently depressed, iv) healthy volunteers with no history of depression. Participants will undergo a clinical assessment, an interview with a trained member of the research team and will complete self-rated questionnaires. Investigators will collect blood and saliva samples to measure certain immune markers. They will also perform magnetic resonance imaging (MRI) scans to look for MRI markers in the brain and investigate brain inflammation in a subsample of these patients using positron emission topography (PET) and cerebrospinal fluid (CSF) sampling (also called lumbar puncture).