Protective proteins in the body aid the immune system in determining which cells are harmful and which are normal and healthy. As the immune system patrols the body for biological troublemakers, it regularly strikes out at normal, healthy cells as well. Cells rely on special protective proteins to avoid being the victims of this friendly fire. Two of these tiny bodyguards--called MCP for membrane cofactor protein and DAF for decay accelerating factor--also may help sperm fertilize eggs and protect sperm in the female genital tract. In addition, scientists are looking for ways to recruit MCP and DAF to tell the immune system what to kill and what to leave alone. This research eventually could affect cancer therapy, organ transplantation, and treatments for a wide range of inflammatory diseases.