They believe their findings will lead to new drugs that target the protein involved so the immune system shrinks tumors without affecting healthy tissue.

Dr. Wayne Hancock of the division of transplant immunology at The Children's Hospital of Philadelphia, and colleagues report their findings in this week's online issue of Nature Medicine.

The study focuses on a question that fascinates immunologists: why doesn't the immune system prevent cancer in the first place? It has the tools to do so.