Danger, danger, cry injured cells.

Lending support to a controversial theory of how the immune system works, researchers have found that injured or dying cells release uric acid, which then stimulates the activity of key immune cells.

Biologists have long described the immune system as something that distinguishes self from nonself, attacking invaders such as infectious microbes but not the body's own tissues. Some investigators have argued that this paradigm is flawed--after all, the human body safely provides a home for many microbes (SN: 5/31/03, p. 344)--and have proposed alternative theories. For example, Polly Matzinger, an immunologist at the National Institutes of Health in Bethesda, Md., suggests that the immune system reacts to microbes only after infected or injured cells have released danger signals.

Uric acid may be one of those signals, Kenneth L. Rock of University of Massachusetts Medical School in Worcester and his colleagues report in the Oct. 2 Nature. They show that cells damaged by heat, chemicals, or radiation increase production of this compound and that it stimulates the activity of the cells that launch an immune response.

The scientists found that the chemical stimulates the immune system only when it reaches concentrations high enough to form tiny crystals. Rock and his colleagues suggest that uric acid could be added to vaccines to boost a body's response to an immunization.--J.T.