A bacterium that tricks the immune system into nurturing it

THE IMMUNE system has many weapons with which to counter hostile incomers. But what works against one may not be effective against another. An interloper can take advantage of this by misdirecting the system into thinking it is fighting an enemy that it is not. This buys time for that interloper to become entrenched. That is sneaky. Sneakier still, though, is the approach just discovered by Ruslan Medzhitov of Yale University. As he and his colleagues report in , they have found a bacterium that induces its host’s immune system to release compounds on which it can then feed.Mammalian immune systems have two modes of attack. Type-1 is used against bacteria and viruses; type-2 against multicellular parasites such as worms. Some invading bacteria, however, provoke a type-2 response when type-1 would be appropriate. Dr Medzhitov decided to take a closer look.