Cytokine, sickness behavior, and depression

Sickness behavior is the adaptive reorganization of the priorities of the host in response to an infectious episode. This process is triggered by proinflammatory cytokines that are produced in response to specific pathogen-associated molecular patterns. Peripherally produced cytokines act indirectly on the brain via a fast neural pathway and a slower humoral pathway. These two pathways combine to produce proinflammatory cytokines in macrophage-like cells and microglia in the brain. The brain cytokine system also can be activated by exogenous cytokines or endogenous cytokines during chronic inflammatory disease. Uncontrolled activation of the brain cytokine system leads to cognitive and mood alterations characteristic of depression.