Examining the benefits of short bouts of everyday stress:

When Dhabhar was starting his graduate work in McEwen’s lab in the early 1990s, “the absolutely overwhelming dogma was that stress suppresses immunity.” But this didn’t make sense to him from an evolutionary perspective. If a lion is chasing you, he reasoned, your immune system should be ramping up, readying itself to heal torn flesh. It occurred to Dhabhar that the effects of acute stress, which lasts minutes to hours, might differ from the effects of chronic stress, which lasts days to years.

Dhabhar likens the body’s immune cells to soldiers. Because their levels in the blood plummet during acute stress, “people used to say: ‘See, stress is bad for you; your immune system’s depressed,’” he says. “But most immune battles are not going to be fought in the blood.” He suspected that the immune cells were instead traveling to the body’s “battlefields”—sites most likely to be wounded in an attack, like the skin, gut and lungs. In studies where rats were briefly confined (a short-term stressor), he showed that after an initial surge of immune cells into the bloodstream, they quickly exited the blood and took up positions precisely where he predicted they would.