The brain's immune cells are increasingly recognized as key players in Alzheimer's disease, but exactly how they change as the disease develops has remained unclear. A new study published in Nature Genetics [DOI: https://10.1038/s41588-026-02716-6] provides the most comprehensive map to date of these cells, identifying a protective subtype of brain immune cell that expands as Alzheimer's disease progresses and uncovering the molecular pathway that enables these cells to help defend the brain.