Dendritic cells, which are responsible for teaching other immune cells to attack infected or mutated cells, face a dangerous predicament. To demonstrate that an enemy has invaded, they must change to look a little bit like the invader. And once they look like an enemy, they risk being treated like one by their newly trained pupils. New research from Rockefeller University shows how the body’s immune system gets around this paradox by using simultaneous signals to both activate and inhibit a killer cell’s immune response.