Dendritic cells are the grand sentinels of the immune system, standing guard 24/7 to detect foreign invaders such as viruses and bacteria, and bring news of the invasion to other immune cells to marshal an attack. These sentinels, however, nearly always fail to respond adequately to HIV, the virus causing AIDS. Now scientists have discovered a sensor in dendritic cells that recognizes HIV, spurring a more potent immune response by the sentinels to the virus.