An unexpected new role hasbeen identified for the immune system's mast cells: They provide protection from snake and honeybee venom (Science 2006, 313, 526). Stanford University pathology professor Stephen J. Galli and colleagues uncovered this behavior when they injected mice with venom. Conventional wisdom suggested that the venom would lead mast cells in the mice to release compounds that cause inflammation, clotting abnormalities, and shock, all of which contribute to tissue injury and death. As a result, mice deficient in mast cells should be more resistant to venom than normal mice. Instead, the researchers found that the deficient mice were much more vulnerable to venom. The researchers discovered that mast cells in mice respond to venom by pumping out carboxypeptidase A and possibly other proteases. These enzymes then break down sarafotoxins and other dangerous substances in the venom. If human mast cells act in a similar manner, the work could lead to improved antivenoms.