NEW YORK (Reuters Health) - The stress of unemployment may dampen healthy people's immune system function -- but the good news is that finding a job can restore its fighting power, according to a new study.

Past research has linked chronic stress to impaired immune function and a higher risk of infection and other illness. But little is known about what happens to the immune system when the stress subsides.

In the new study, researchers at the University of California San Francisco followed 100 employed and 100 unemployed adults between the ages of 29 and 45 for 4 months.

The employed subjects were matched to the unemployed subjects for factors such as gender, age, race and education level. Individuals with chronic conditions or on medical treatments that could affect the immune system, smokers and intravenous drug users had been excluded from the study.

Each month, the researchers collected blood samples from the participants to measure levels of "natural killer" cells, an indication of the strength of their immune system.

The researchers found that, in general, the unemployed group had weaker natural-killer activity than the working group. However, 25 percent of the unemployed participants found jobs during the study period, and their natural killer cells subsequently got back on track.

"We believe this is the first study in humans to document immune function recovery after the definable end of a chronic stressor," Dr. Frances Cohen and her colleagues report in the journal Psychosomatic Medicine.

Chronic stress may impair immune function through its effects on the nervous system, according to the researchers. The adrenal glands release the hormone norepinephrine in response to stress, and in the test tube, norepinephrine has been shown to reduce natural killer cells.

But the current findings suggest that immune function is "resilient" in the face of long-term stress and can quickly recover once the stressor is gone, Cohen's team points out. Study participants' immune system recovery began within the first month of their new employment, the study found.

Further studies, according to Cohen's team, should investigate immune system recovery from different types of stressors, and try to figure out how that comeback happens.