Stress is supposed to be bad for your health, right? Generally, that's the advice delivered by doctors and scientists.
But occasional, short bursts of stress can actually be good for the immune system, and in a new study of rats, Stanford University researchers offered new insights into how the body gets a boost when its "fight or flight" system is turned on every now and then.
The connection between stress and the immune system boils down to how key stress hormones mobilize and get to organs such as the skin, which could be injured if you are attacked.
This response is important since the cells of the immune system are vital for healing wounds or killing infectious bacteria.
Firdaus Dhabhar, the study's lead author and an associate professor of psychiatry and behavioral sciences at Stanford University, said the study's findings describe the body's finely coordinated system to detect danger and prepare to protect itself.
"The immune system on its own doesn't know that a lion may be about to chase the person or that the person is undergoing surgery. But the brain does. We believe the brain, through the release of stress hormones, is preparing the immune system to deal with those challenges," he said.
The scientists studied rats that were briefly confined in ventilated, Plexiglass enclosures. Then they took blood samples from the rats over the next two hours after the stressful situation, looking for amounts of stress hormones as well as specific disease-fighting agents of the immune system.
They found that the rats' bodies released three key stress hormones -- norepinephrine, epinephrine, and the rat version of cortisol -- in stages. The hormone cycles worked to bring immune cells out of the spleen and bone marrow, into the bloodstream and eventually to the skin.
The study was published Thursday in the Journal of Psychoneuroendocrinology.
Although the scientists were studying rats, humans have similar responses to stress. Dhabhar said the stress of surgery may actually be helpful in helping people heal after their operations.
The study's findings make sense when you look at them in the light of human evolution, said Dr. Redford Williams, director of the Behavioral Medicine Research Center at Duke University.
"Our ancestors probably needed to have a robust immune system to fight off germs that would get into their bodies when they got slashed by a saber tooth tiger, a highly stressful situation," he said.
But stress is only good in moderation, perhaps for a few minutes to a few hours at a time. If the body deals with stress over weeks, months or years, the results can be unhealthy.
Scores of studies have found that long-term stress aggravates many common conditions, such as heart disease, depression and gastrointestinal maladies, nullifying the initial benefits to the immune system.
"If you're stressed day in and day out for a long time, you're really beginning to damage yourself as well as the germs," Williams said.