Cytokine theory of depression

The cytokine theory of depression posits that inflammation — elevated peripheral pro-inflammatory cytokines — is a causal contributor to a substantial subset of major depressive disorder. It is not merely that depressed people have inflamed bodies; it is that the inflammation is doing the depressing.

The evidence is experimental, not merely correlational. Administration of interferon-alpha — a cytokine used to treat hepatitis C — produces depressive symptoms in 30-50% of patients within weeks. Conversely, anti-cytokine therapies, including monoclonal antibodies against tumor necrosis factor-α and interleukin-6, show antidepressant effects in patients with treatment-resistant depression and elevated inflammatory markers. The sickness behavior induced by acute infection and the symptoms of clinical depression are not analogues. They are the same neuro-immune program operating on different timescales.

The theory challenges the disciplinary boundary between psychiatry and internal medicine. If depression can be caused by peripheral inflammation, then it is not fundamentally a disorder of cognition or mood — it is a disorder of neuro-immune regulation that happens to present with psychiatric symptoms. The implication is radical: some depressions may be more effectively treated by immunologists than by psychiatrists.