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Immune cells secrete cytokines (shown as red dots in this simplified drawing, click on image to see larger version) that trigger inflammatory responses. But when cytokine levels in the brain stay high fo too long, people susceptible to mood disorders may develop depression. Cytokines released in the body may enter the brain directly, by passing through leaky areas in the blood-brain barrier, or indirectly by initiating a chain reaction of "middlemen" that lead to brain cells called microglia releasing cytokines. Cytokines may alter mood by changing brain processes and levels of brain chemicals such as serotonin and dopamine. Synthetic version of interferon-alpha and interferon-beta, used to treat cancers, hepatitis C and multiple sclerosis, may engage the same pathway. Credit: Amadeo Bachar