In the long effort to understand why the brain deteriorates in Alzheimer's disease and related conditions, science has now traced an unexpected detour through the immune system — one that begins not in the brain, but in the lymph nodes that drain it. Researchers have found that specialized immune sentinels called dendritic cells may be quietly training other immune cells to enter the brain and worsen the damage already caused by toxic tau proteins. The discovery, still unconfirmed in humans, opens a door toward therapies that might interrupt this hidden feedback loop before it accelerates neur