Scientists have discovered a new mechanism controlling the choice in humans between two lines of defense in the event of attack. In the presence of viruses or bacteria, the immune system can trigger a response that is rapid but devoid of memory — innate immunity — or a response that takes longer to put in place but is more specifically targeted — adaptive immunity. The body is often faced with attacks from outside (viral or bacterial infection) and sometimes from inside, because of the dysfunction of its own cells (cancer), and defends itself by activating its immune system.