A short burst of stress appears to help the body fight off infections, but chronic stress may produce the opposite effect, according to a new report.

After reviewing 300 studies that investigated the link between stress and immunity, researchers found that short-term stress appears to rev up the immune system, while chronic stress produces changes in the body that seem to diminish immune functioning.

So are people who are under stress for months at a time - a result of unemployment, for instance - more prone to illness? Unfortunately, more research is needed to before researchers can make that conclusion, study author Dr. Suzanne C. Segerstrom cautioned.

This question is “not easy to answer from this body of research,” she said.

Segerstrom and her co-author, Dr. Gregory Miller, based their report on a review of 293 studies conducted between 1960 and 2001, in which almost 19,000 people took part.

The researcher, who is based at the University of Kentucky in Lexington, explained that when we are stressed, our bodies release stress hormones, such as adrenaline and cortisol. When these hormones are intermittently present during brief periods of stress, they cause the body to release immune cells capable of quickly and efficiently fighting off infections.

“In essence, you’re getting ‘first responders’ on the scene, in case something happens,” Segerstrom told Reuters Health.

However, the picture is quite different once those stress hormones are present for prolonged periods, the authors report in the journal Psychological Bulletin.

Segerstrom explained that our bodies carry immune cells that respond only to specific triggers, such as one virus or bacterium. We don’t have enough room to carry legions of each type of these cells, so when a particular trigger is present, the cell that targets that trigger makes multiple copies of itself and responds.

However, when people are under prolonged periods of stress, these trigger-specific cells don’t multiply as well, thereby reducing their ability to fight the triggers, she said.

In an interview, Segerstrom explained that it makes sense that humans would develop a system that enables their bodies to fight off infections during short bursts of stress. When early humans were running from danger, for instance, they were more likely to get injured and a subsequent infection, and that short-term boosting of their immune system was likely a good source of protection, she said.

However, the change in immune system functioning with long-term stress “does not make as much sense,” she said, and is likely an “unintended consequence” of chronic stress, which is relatively new, in evolutionary terms.

Long-term stress, such as from unemployment or poverty, “was just not characteristic of the kinds of things people experienced, until very recently,” said Segerstrom.

SOURCE: Psychological Bulletin, July 2004.