Most people feel down, tired and inactive when they’re injured or ill. This “sickness behaviour” is caused by the activation of the body’s immune response. It’s the brain’s way of conserving energy so the body can heal.
This immune response can also occur in people with depression. This has prompted some researchers and clinicians to hypothesise that depression is actually a side effect of the inflammatory process.
But while there may be a connection between inflammation and depression, one doesn’t necessarily lead to the other. So it’s too simplistic to say depression is a physical, rather than a psychiatric, illness.
The Inflammation Hypothesis
University of California clinical psychologist and researcher George Slavich is one of the key recent proponents of depression as a physical illness. He hypothesises that social threats and adversity trigger the production of pro-inflammatory “cytokines”. These are messenger molecules of the immune system that play a critical role in orchestrating the host’s response to injury and infection.
This inflammatory process, Slavich argues, can initiate profound behavioural changes, including the induction of depression.
The idea that the activation of the immune response may trigger depression in some people is by no means a new one. Early descriptions of post-influenza depression appeared in the 19th century in the writings of English physician Daniel Tuke.
But it was not until the 1988 seminal paper, published by veterinarian Benjamin Hart, that the phenomenon of acute “sickness behaviour” caught the interest of the scientific community.
