January 24, 2008 in Health/Science -- Demi-Lee Brennan was born with O-negative blood, but doctors say her body spontaneously switched to O-positive after she received a new liver.

"I was convinced we had made a mistake, so we tested it again and it came up the same," Dr. Julie Curtin tells The Age. "Then we tested her parents and they were both O-negative, so it was confirmed that Demi absolutely had to have been O-negative."

Doctors tell AFP that Brennan, who was nine at the time of the transplant, developed an post-operative infection that may have allowed stem cells from the donor's liver to enter her bone marrow. "In effect she had had a bone marrow transplant. The majority of her immune system had also switched over to that of the donor," Dr. Michael Stormon tells the French news agency.

Brennan, 15, doesn't have to take any of the anti-rejection drugs that are routinely prescribed to transplant patients. There's one catch: ABC News says she lost all of her antibodies, meaning that they had to re-administer all of the vaccines she received as a baby and young child.

"I'm probably the most grateful person because that has saved my life, that gave me a chance to fulfill my life," Brennan, an aspiring musician, tells ABC News. "I just want to live it the most I've got for them and to show them that I'm so grateful."

The doctors describe it as the "holy grail" of transplants. "We consulted widely throughout the hospital and then looked at the medical literature and consulted colleagues around the world to see if anyone had seen this kind of thing before and no one had, so we were stunned and amazed," Dr. Michael Stormon of Children's Hospital in Sydney tells ABC Radio Australia.

Kerrie Mills, the girl's mother, tells The Age that, "She's my little hero." Our heroes are the anonymous parents who donated their 12-year-old son's organs after he died of brain cancer.