In a mice study, mice whose stress response involved overproducing an inflammatory compound called interleukin-6 were more likely to become depressed than mice who didn't overproduce the compound. Study researcher Georgia Hodes, of Mount Sinai Medical Center in New York, notes that the same compound is developed in depressed humans.

"There's probably a subset of people with depression who have this over-sensitive inflammatory response to stress and... this is leading to the symptoms of depression," Hodes told health news website LiveScience.

"In some ways, it is an analogy to an allergy," she said. "You have something that is not really dangerous, but your body thinks it is, so you have this massive immune response. In this case, the stressor is what they're having this massive immune response to."

To further test their theory, the research team also transplanted the bone marrow of depressed mice into healthy mice and found that the previously healthy mice became depressed after experiencing a mild stressor. "They also found that mice with immune cells that release more interleukin-6 in response to a toxin developed a more severe depression-like response to the stress," noted a press release.

The researchers are reporting their findings on October 16 at the annual meeting of the Society for Neuroscience in New Orleans.

According to LiveScience, some of the drugs used in the study to lessen the immune response are already currently used to treat rheumatoid arthritis, which means they could easily be tested to treat depression.