[Major depressive disorder: factors of risk, vulnerability and protection].

Major depressive disorder is a disease whose course is often chronic because the prevalence of recurrence is high. Therefore, it is important to know the risk factors of relapse (number of previous episodes, persistence of residual depressive symptoms…). Moreover, identifying factors of vulnerability of the disease may help to detect very-high-risk subjects in order to prevent the disease onset. Many hypothesis are explored: serotonin dysfunction, axis of stress, alteration of brain structures. Finally, antidepressant treatment is a protective factor and is efficient to limit the risk of chronicity. Copyright Â