Enlarge Image Looking for trouble. Cytotoxic T cells like this one might react to distress signals released by dying or damaged body cells. Credit: Eye of Science/Photo Researchers Inc. In 1994, Polly Matzinger came up with a controversial idea. The immunologist at the U.S. National Institute of Allergy and Infectious Diseases proposed that alarm signals released from injured and dying cells can kick our immune system into high gear even when no microbial threat is evident. Many of Matzinger's colleagues ridiculed her "danger hypothesis," and it has remained divisive ever since. But a new study lends strong support to the...