In the quiet transparency of a zebrafish larva, scientists in Dresden have glimpsed something that has eluded medicine for generations: a molecular signal that tells the immune system to heal rather than destroy. A specific class of neutrophils, long regarded as mere foot soldiers of inflammation, has been found to release a molecule called Il-4 that orchestrates nerve regeneration after spinal cord injury. The discovery does not yet promise a cure, but it names, for the first time, a conductor in the biological symphony that separates animals that recover from those that do not.