Stress is bad for you -- you've heard it a thousand times. But it can be good for your immune system, says a study.

Short-term stress, the fight-or-flight response, a mobilisation of bodily resources lasting minutes or hours in response to immediate threats -- stimulates immune activity, said Firdaus Dhabhar, associate professor of psychiatry and behavioural sciences and member of the Stanford University Institute for Immunity, Transplantation, and Infection.

And that's a good thing. The immune system is crucial for wound healing and preventing or fighting infection, and both wounds and infections are common risks during chases, escapes and combat, the Journal of Psychoneuroendocrinology reported.

Working with colleagues at Stanford and two other universities, Dhabhar showed that subjecting lab rats to mild stress caused a massive mobilization of several key types of immune cells into the bloodstream and then onto the skin and other tissues, according to a university statement.

This large-scale migration of immune cells, which took place over two hours, was comparable to the mustering of troops in a crisis, Dhabhar said. He and colleagues had previously shown that a similar immune-cell redistribution in patients experiencing the short-term stress of surgery predicts enhanced postoperative recovery.

Investigators were able to show that the massive redistribution of immune cells throughout the body was orchestrated by three hormones released by the adrenal glands, in different amounts and at different times, in response to the stress-inducing event. These hormones are the brain's call-to-arms to the rest of the body, Dhabhar said.

"Mother Nature gave us the fight-or-flight stress response to help us, not to kill us," said Dhabhar, who has been conducting experiments for well over a decade on the effects of the major stress hormones on the immune system.

The findings paint a clearer picture of exactly how the mind influences immune activity. "An impala's immune system has no way of knowing that a lion is lurking in the grass and is about to pounce, but its brain does," Dhabhar added.