In a meta-analysis, Howren MB discovered that the levels of inflammation factors, such as C-reactive protein (CRP), interleukin-1β (IL-1β) and interleukin-6 (IL-6), in depressive patients were significantly higher than those in patients without depression and that the depressive patients often showed depressive symptoms after being given inflammatory cytokines, such as interferon-α3. Here, CRP is linked to major depressive disorder.