Life's stresses, both the day-to-day and life-changing events, take their toll on the body's immune system, degrading it over time. But there's evidence that short bursts of stress, as long as they have a known end, can actually be good for the immune system, because they jolt it into higher activity. Two psychologists from the University of Kentucky and the University of British Columbia dug up more than 30 years of studies on stress and the immune system to figure out which stresses are good and which are bad for our bodies.

What the researchers wanted to know: How do different types of stress affect the immune system?

What they did: The researchers compared data from 293 studies on all different types of stress—people asked to do mental math in lab tests, people who had lost a spouse, and refugees running from their homeland. They divided types of stress into categories based on the severity of the situation and how long the stress continued. Then, they looked at the effect of different types of stress on specific immune responses, such as the percentage of immune system cells in the blood.

What they found: When people were stressed over a long amount of time, in situations that changed their lives dramatically, their immune system usually became degraded, making them more susceptible to illness. However, in short bursts, such as a public-speaking event, when people knew that the stress had a definite end point, stress actually boosted immune response. The authors hypothesize that this effect is a remnant of the primitive fight-or-flight response, which temporarily beefs up the body's defenses to allow it to challenge, or run from, danger.

What it means to you: Stress isn't something we seek out, but this study suggests it's not bad to face a situation that gets our hearts racing a little bit. The authors compared the process of becoming stressed, completing a task, and then relaxing afterward to a workout for the immune system‑because cells that protect your body from infection are mobilized throughout your body. But, they cautioned that being stressed and anxious for long periods of time still has a very harmful effect.

Caveats: Because the authors relied on published data, there is a chance that their results would be biased toward studies that resulted in larger effects than are actually there, since studies that don't find a big effect are less likely to get published.. However, the authors tested the reliability of their data and drew from a wide range of sources to mitigate that possibility.

Find out more: The American Institute of Stress has a good website at (what else) http://www.stress.org. The National Institutes of Health has information about the links between stress and other health problems such as alcohol abuse and disease at http://health.nih.gov/.

Read the article: Segerstrom, S.C. and G.E. Miller. "Psychological Stress and the Human Immune System: A Meta-Analytic Study of 30 Years of Inquiry." Psychological Bulletin. July 2004, Vol. 130, No. 4, pp. 601–630.