ARE ANIMAL ORGANS SAFE FOR PEOPLE?

JEFF GETTY SHOULD BE DEAD BY NOW. He has had HIV for about 15 years. His immune system is barely functioning. And on top of that, in a desperate attempt four weeks ago to reverse the course of his disease, doctors at San Francisco General Hospital infused him with an experimental bone-marrow transplant from a baboon. Immunologists warned that his body would eventually reject the nonhuman tissue and that the operation would almost certainly end his life rather than prolong it. However, Getty is not only alive, but last week he was healthy enough to go home from the hospital. No matter how much time he has left, friends and family call him a medical miracle.

But there is also a small chance that the technology that allows people like Getty to receive tissues from animals could someday unleash a medical disaster. The danger is that patients could receive a previously unknown microbe along with their transplants. When viruses or bacteria have made the jump from animals to humans in the past, they have often proved exceedingly virulent: HIV, which causes AIDS; Ebola virus; and hantavirus are all chilling precedents. In a worst-case scenario, such transplants could introduce humanity to a plague that would make all of those look tame. "This is a serious mistake," says Jonathan Allan, a virologist at the Southwest Foundation for Biomedical Research in San Antonio, Texas. "It only takes one transmission from one baboon to a human to start an epidemic. There's no way you can make it safe."

No one ever had to worry about such potential hazards before, because scientists hadn't been able to figure out how to make an animal-to-human transplant work. It's hard enough to trick an individual's immune system into accepting tissues from another person. But when organs from an entirely different species are stitched into the human body, immune defenses go into overdrive, leading to swift and irreparable destruction of the foreign tissue. Two years ago, when doctors at the University of Pittsburgh transplanted baboon livers into two seriously ill patients, both men died soon after the operation.

The advent of genetic engineering, among other things, has allowed researchers to begin breaching that natural barrier. Last spring scientists at Duke University Medical Center reported that they had successfully altered the genetic makeup of a strain of pigs. As a result, the researchers managed to fool the immune systems of three baboons into accepting pig hearts, for a short while at least. Using a similar technique, the British biotechnology company Imutran has produced a herd of 300 genetically altered swine. The company expects to transplant either a pig's heart or a liver into a human subject later this year.