If bad bacteria lurk in your system, chances are they will bump into the immune system’s protective cells whose job is gobbling germs. The catch is that these do-gooders, known as macrophages, ingest and destroy only those infectious invaders that they can securely hook and reel in. Now, scientists have shown that a healthy immune response depends on a protein called TRPV2 which, they discovered, is the means by which macrophages capitalize on brief and accidental encounters with nasty bugs.