A mysterious bowel disease thought to be caused by an over-exuberant immune system may paradoxically be triggered by immune cells that don't do enough in the early stages of bacterial infection.
Since some treatments for Crohn's disease aim to suppress the immune system, it's possible these drugs could be making things worse. The discovery by Anthony Segal of University College London and his colleagues is causing a stir among immunologists. Caetano Reis e Sousa at Cancer Research UK calls it "provocative", while Jean-Laurent Casanova at The Rockefeller University in New York says it is "a major breakthrough".
A similar mechanism may be at the root of a host of other "autoimmune" disorders, in which immune cells turn on the body's own tissue. Underactive immune cells could also explain why some of us are more prone to infectious diseases.