Monika Fleshner, a neuroimmunophysiologist at the University of Colorado, Boulder, has shown that short-term stress can even boost immunity. It's one of the body's natural defense systems. So, if the immune system is indeed critical to how the body processes chronic stress, short-term stress might help build the body’s defenses against stress itself.

Brachman doesn’t want to completely cut off a person’s stress response, as that could prove counterproductive. Instead, she’s seeking a way for people to stop the response before it turns dangerous—before it triggers mental illness. It’s a mission she landed on somewhat accidentally while completing a fellowship at the National Institutes of Health, where she researched how the immune system—specifically, a compromised immune system—is related to depression. She was particularly interested in whether depression influenced the immune system, or if the immune system helped trigger depression.

Working with 12 mice, her team transferred immune cells from depressive-like mice into new mice that had not been exposed to stress or displayed depressive behaviors. The team expected the new mice either to develop a typical physiological response to stress (inflammation, increased heart rate) or to show no changes. But when they actually stressed the new mice—by putting them in cages with more aggressive animals, forcing them to swim, and other standard lab methods—the results, and those of subsequent studies, showed the opposite: The new mice were more resilient to stress.

The immune cells from the depressed mice appeared to immunize the new mice against stress-induced depressive symptoms, as if they were a vaccine.

By the time Brachman’s results were published, in January 2015, she’d long since moved on to and completed a doctoral program at Columbia University, where she had begun collaborating with the neurobiologist Christine Denny.* In the course of her career, Denny had seen thousands of mice anesthetized with ketamine, the drug also known as special-K, for various studies, and recognized that there were lasting effects. Ketamine was by then also being used to treat depression in adults. Brachman and Denny decided to test whether ketamine had any effect on how mice react to stress. They gave the drug to lab mice, waited one week, then put those mice, as well as untreated mice, through stressful situations. The study results showed that for up to four weeks after the injection, the mice that were given ketamine were more social and less afraid than the control animals. They also exhibited fewer depressive-like symptoms after facing stressful situations.

Brachman’s results consistently showed that there are ways to build resilience to stress, by both injecting immune cells from another animal and giving a dose of ketamine prior to stress. This runs counter to the long-accepted notion that stress leads to illness because of risk factors such as genetic mutations, compromised immune systems, and prior exposures. Perhaps the relationship between stress and illness has more to do with a person’s level of resilience—and perhaps specific interventions could boost this.