Stress from society, work, or life tend to cause elevated levels of proinflammatory cytokines such as IL-1beta, IL-18, IL-6, and TNF-alpha in both the periphery and brain, leading to sickness behavior syndrome; after treatment with antidepressants, the elevated levels of proinflammatory cytokines in some depression patients return to normal. Here, IL6 is linked to major depressive disorder.