The true cause of depression remains unclear, but many experts suggest that chemical imbalances in the brain or genetic disposition are largely to blame. However, a growing body of intellectuals suggest a different and far more shocking culprit: Our own immune system.

According to the new theory, for some patients, depression may be a result of an errant immune system causing inflammation in the body and altering mood. This theory may explain why many patients have consistently high levels of inflammation, suggesting that inflammation is not a side effect of depression, but rather the cause of it, The BBC reported. If this theory proves true, it could also offer new, more effective ways to treat patients that have not responded to antidepressant medications or therapy.

This is not the first time that scientists have proposed the idea that inflammation could play a role in the underlying causes of depression. For example, in a 2015 study, researchers found that people with clinical depression had a 30 percent increase in brain inflammation, also referred to as neuroinflammation. The authors took brain scans of 20 patients who were suffering from depression but were otherwise healthy, as well as of 20 control patients. They found that people with depression were more likely to suffer a higher rate of inflammation in their brains. What’s more, people with the most severe depression had the highest rates of all.

In a recent news story on the the theory, the BBC spoke with Dr. Iain McInnes, a consultant rheumatologist at Glasgow Royal Infirmary. McInnes explained that, in his experience in rheumatology, a profession focused on addressing inflammation of the joints, muscles, and ligaments, he has seen an almost disproportionate increase in well-being and mood in patients who had received anti-inflammatory treatment for their arthritis.

"We scanned the brains of people with rheumatoid arthritis, we then gave them a very specific immune targeted therapy and then we imaged them again afterwards,” McInnes told the BBC. "What we are starting to see when we give anti-inflammatory medicines is quite remarkable changes in the neuro-chemical circuitry in the brain.”

For now, the theory remains just that, a theory. Currently, researchers are working on obtaining more evidence in support of this hypothesis. Ultimately, the goal is to find a new treatment for depression and develop a test to identify those who will benefit. Although it will likely be many years before this goal is reached, there is one silver lining. Many of the potential drug candidates have already been FDA-approved for other uses, making the drug testing process slightly easier.

"We think by measuring inflammation in the blood we'll actually be able to identify individuals that do require more complex, intensive antidepressant treatment, maybe a combination of an antidepressant and and anti-inflammatory," Carmine Pariante, a professor of biological psychiatry from King's College London, told the BBC.

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