Narrator: This is Science Today. When someone gets an organ transplant, such as a heart or kidney, their immune system tries to reject the new organ as it would a splinter or any other foreign body. Transplant surgeon Flavio Vincenti of the University of California, San Francisco says the only way to deal with organ rejection is with drugs that suppress the immune system.

Vincenti: And the major problem that we face is that these drugs are to some degree non-selective. They depress the immune system across the board.

Narrator: Which makes the patient prey to all kinds of infections. But if you don't give enough drugs, the immune system rejects the organ.

Vincenti: So you have to use higher doses of medications...

Narrator: Thus damaging the new organ irreversibly. So transplant doctors are looking for a drug that suppresses only the part of the immune system that attacks the new organ, leaving the rest alone. An experimental drug called HAT seems to do just that. In initial tests, it seems safe, so Vincenti and colleagues at 20 hospitals worldwide are testing HAT more thoroughly to see if it really prevents rejection. For Science Today, I'm Steve Tokar.