'A Piece in the Jigsaw': New Theory Links Depression to Immune Disorders

Inflammation, the main weapon of the immune system, seems to have a direct effect on the brain and could even be causing depression, according to recent research on the link between physical illness and depressive disorder.

Depression may be caused by an overactive immune system which causes inflammation throughout the body, making people feel down and fatigued, according to several new studies into the connection between depression and physical illness.

In fact, for decades scientists have known of a link between depression and inflammatory disorders such as rheumatoid arthritis, heart disease and hepatitis. These patients are 8 to 10 times more likely to suffer from depression than the general population, Dr. Alan Carson, a professor in Neuropsychiatry at the University of Edinburgh, told Radio Sputnik.

"We also know that if you have depression and one of these illnesses you're more likely to have a bad outcome, including death, so there's always been known to be some link there. The flip side of that is that is that these illnesses are treated with a variety of immunosuppressant agents, and some of them seem to directly cause depressive illnesses," he explained.

"What's become increasingly clear over the last 20 years, from different strands of scientific evidence, is that it's not just that people are feeling miserable because they've got rheumatoid arthritis and their joints are sore, but there seems to be a direct brain effect."

When it feels under threat, the immune system unleashes cascades of molecules called cytokines and interleukins, which not only fight off illness but also have an effect on the hypothalamus and amygdala in the brain. The hypothalamus is responsible for the production of many of the body's essential hormones, while the amygdala get characterized as the brain's "panic center."

"It [the immune system] may act by increasing the stress response hormone, the so-called corticotropin-releasing hormones which make people feel stressed … or it might act directly on a chemical called alpha-interferon, which reduces the production of serotonin on the brain."

Hypotheses about the origin of depressive illness with the immune system are not seen as alternate explanations to the more traditional view of depressive illnesses as connected to alterations in the neurotransmitter serotonin. Rather, they are "pieces in the jigsaw" which help understand why some people are suffering from alterations in serotonin metabolism or the stress hormone cortisol.

"It's really more that we've got this very large jigsaw and we're just starting to put a few more of the corner pieces in and build up the picture of why it's happened," Carson said.