In our field of research, this social stress test is the most effective way to induce a neuroendocrine stress response. And we also know it induces a pro-inflammatory response in humans that is exaggerated in people who have major depressive disorder, for example, Lowry told Gizmodo.

However, the study also had some counterintuitive results. Farm-raised men, on average, reported that they felt more stressed by the task than city men did, and they actually had higher levels of cortisol, a hormone indicative of stress, in their saliva. This seemingly contrary finding could be explained by the fact that our reactions to stress can manifest through interconnected bodily systems that work independently of one another. Cortisol is a key component of the hypothalamicpituitaryadrenal axis, a pathway that runs along our brain and hormonal glands that regulates everything from digestion to our flight-and-fight reaction. The presence of PBMCs and interleukins brought on by an overactive immune response to stress might not affect cortisol levels, and vice versa.

There are important limitations to the study, such as a small sample size and lack of women subjects, who are more likely than men to report mental illness. The connection between where we grow up and our later mental health might also have more to do with the well-documented psychological benefits of living near nature than its direct effect on our immune system, Lowry said.