Is there a link between inflammation and mental health?

Multiple lines of evidence have implicated increased brain inflammation (neuroinflammation) in mental health disorders. One example is autoimmunity, where inappropriate targeting of brain proteins by the immune system can increase neuroinflammation, negatively affecting brain function.^[1] Neuroinflammation isn’t always caused by autoimmunity, however, since increased systemic levels of pro-inflammatory cytokines, such as interleukin-1 beta (IL-β) and tumor necrosis factor-alpha (TNF-α), have been shown to cause social withdrawal, triggering “sickness behavior”^[2], a mechanism that may have evolved to encourage social isolation to decrease transmission of communicable diseases. While the mechanisms by which inflammation induces sickness behavior aren’t completely understood, experiments with lipopolysaccharide (LPS, an inflammation-inducing component from the cell wall of certain bacteria) have shown that increased levels of inflammation can have direct effects on the brain in humans. In one study where healthy participants were injected with low doses of LPS, reward signaling in the brain was suppressed, causing anhedonia (the inability to feel pleasure), a common symptom in people with depression.^[3]