Increased mean levels of peripheral inflammatory markers have repeatedly been reported in individuals with major depression compared to controls, although there is considerable overlap between groups. As further evidence for a role of inflammation in major depression, 20-60% of patients receiving treatment for viral hepatitis and certain forms of cancer with the cytokine interferon-alpha will develop depressive symptoms. Interestingly, some authors have suggested that the association between inflammation and depression is symptom specific, i.e. there might be a subtype of "inflammation-related depression" with a specific phenotype. Although it still remains unclear if the immune abnormalities often seen in depressed subjects derive from changes in the periphery or in the central nervous system (or both), animal studies have shown that by counteracting the effects of pro-inflammatory cytokines in either the blood or the brain, depressive-like behavior in animals can be mitigated. These animal studies, along with data from several clinical studies pertaining to this, suggest that interventions primarily targeting peripheral inflammation may be useful in treating psychiatric symptoms. Although the exact mechanisms by which increased inflammation may give rise to depressive symptoms remain unclear, there are several potential downstream pathways that may be involved including accelerated cellular aging, mitochondrial dysfunction, and oxidative stress.