Sept. 25 (HealthDay News) -- Periods of short-term stress boost the immune system and protect against a certain type of skin cancer in mice, U.S. researchers say.

The finding was surprising, the researchers noted, because it's believed that chronic stress weakens the immune system and increases the risk of disease.

"This is the first evidence that this type of short-lived stress may enhance anti-tumor activity," Firdaus Dhabhar, an associate professor of psychiatry and behavioral sciences at Stanford University School of Medicine, and a member of Stanford's Cancer Center and Institute for Immunity, Transplantation and Infection, said in a news release from the university.

"This is a promising new way of thinking that calls for more research," Dhabhar said. "We hope that it will eventually lead to applications that help us to care for those who are ill by maximally harnessing the body's natural defenses while also using other medical treatments."

In the study, mice were exposed for 10 weeks to doses of cancer-causing ultraviolet light. Some of the mice were subjected to nine periods of short-term stress by placing them in a confined space that limited their ability to move. Each stress session lasted 2.5 hours, the authors explained.

Compared with non-stressed mice, fewer of the acutely stressed mice developed a type of skin cancer called squamous cell carcinoma during weeks 11 through 21. The stressed mice that did develop skin cancer had few tumors than the non-stressed mice.

But the protective effect of the acute stress wasn't permanent, the researchers found. After week 22, about 90 percent of mice in both groups developed cancer, but the stressed mice continued to have fewer tumors until week 26.

"It's possible that the pre-tumor cells were eliminated more efficiently in the group that was stressed," Dhabhar said. "There may also have been a longer-term enhancement of immunity, as we have seen in our non-cancer-related studies. However, acute stress did not lower tumor burden beyond week 26. We are in the process of determining why."

The findings were released online Sept. 16 on the Web site of Brain, Behavior, and Immunity.

SOURCE: Stanford University, news release, Sept. 21, 2009