Stress is good for your body, especially if you're in danger of being injured or catching an infection.

Contrary to popular belief, scientists found in a study on rats that immune responsiveness is enhanced rather than suppressed by the so-called "fight-or-flight" response.

While chronic stress, which lasts from weeks to months, is bad for health and can suppress immune response, short-term stress triggers the body's "fight or flight response," a mobilization of bodily resources that lasts from minutes to hours in response to immediate threats, and activates immune activity, according to lead author Firdaus Dhabhar, an associate professor of psychiatry and behavioral sciences and member of the Stanford Institute for Immunity, Transplantation, and Infection.

Researchers said that heightened immune responsiveness enhances the body's ability to heal and prevent wounds and infections, injuries that are common during chases, escapes and combat.

In the study, published online June 22 in the Journal of Psychoneuroendocrinology, Dhabhar and his research team subjected laboratory rats to mild stress by keeping them in transparent Plexiglas enclosures. 

Researchers had taken blood samples several times over a two-hour period, and measured levels of three hormones norepinephrine, epinephrine and corticosterone (cortisol in humans) as well as several distinct immune cell types in the blood.

Researchers found a pattern of carefully choreographed changes in the levels of three hormones and different levels of immune cells from reservoirs in places like the spleen and bone marrow.

To find out what each hormone is responsible for, researchers administered the three hormones, separately or in various combinations, to rats whose adrenal glands had been removed so they couldn't generate their own stress hormones, and compared the immune-cell migration patterns of the previously confined rats to the ones without adrenal glands.

Dhabhar found that norepinephrine is released early and is primarily involved in mobilizing all major immune-cell types like monocytes, neutrophils and lymphocytes. Epinephrine, which was also released early, mobilized monocytes and neutrophils into the blood, while helping lymphocytes get out into "battlefield" destinations such as skin. Corticosterone was released somewhat later and caused almost all immune cell types to head out of circulation to the "battlefields."

Researchers compared this large scale migration of immune cells, which happened in the course of two hours, to the deployment of military troops in a crisis. 

In a previous study, Dhabhar and his team found that that similar immune-cell redistribution in patients experiencing short-term stress of surgery predicted enhanced postoperative recovery.

The latest findings showed that the massive redistribution of immune cells throughout the body was caused by the three hormones released by the adrenal glands, in different amounts and at different times, when faced with a stress-inducing event.

Experts likened the three hormones as the brain's call-to-arms to the rest of the body.

"Mother Nature gave us the fight-or-flight stress response to help us, not to kill us," Dhabhar, who has been conducting experiments for over 10 years on the effects of the major stress hormones on the immune system, said in a statement.

Dhabhar said that the latest findings provide researchers with a clearer picture of 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," he said.

Dhabhar said that both lions and impalas benefit from heightened immune response caused by short-stress because pathogen-fighting immune cells are in positions of readiness in the skin and mucous membranes, parts of the body most at risk for damage and consequent infection.

Researchers said that the latest findings make perfect evolutionary sense that  predator or prey activity and other situations in nature, such as dominance challenges and sexual approaches, trigger stress hormones.

"You don't want to keep your immune system on high alert at all times," Dhabhar said. "So nature uses the brain, the organ most capable of detecting an approaching challenge, to signal that detection to the rest of the body by directing the release of stress hormones. Without them, a lion couldn't kill, and an impala couldn't escape."

Not only do stress hormones protect the body, they can also make animals stronger, helping them run faster, jump higher and bite harder.