CANBERRA, Jan 25 (Reuters) - An Australian teenage girl has become the world's first known transplant patient to change blood groups and take on the immune system of her organ donor, doctors said on Friday, calling her a "one-in-six-billion miracle".

Demi-Lee Brennan, now 15, received a donor liver when she was 9 years old and her own liver failed.

"It's like my second chance at life," Brennan told local media, recounting how her body achieved what doctors said was the holy grail of transplant surgery. "It's kind of hard to believe."

Brennan's body changed blood group from O negative to O positive when she became ill while on drugs to avoid rejection of the organ by her body's immune system.

Her new liver's blood stem cells then invaded her body's bone marrow to take over her entire immune system, meaning the teen no longer needs anti-rejection drugs.

Doctors from Sydney's Westmead Childrens' Hospital said they had no explanation for Brennan's recovery, detailed in the latest edition of The New England Journal of Medicine.

"There was no precedent for this having happened at any other time, so we were sort of flying by the seat of our pants," Michael Stormon, a paediatric hepatologist, told local radio.

Stuart Dorney, the hospital's former transplant unit head, said Brennan's treatment could lead to breakthroughs in organ transplant treatment, because normally the immune system of recipients attacked the transplanted tissue.

"We now need to go back over everything that happened to Demi-Lee and see why, and if it can be replicated," said Dorney.

"We think because we used a young person's liver and Demi-Lee had low white blood cells, that could have been a reason," he told the Daily Telegraph newspaper.

Rejection is normally treated with a combination of drugs, although chronic rejection is irreversible.

Only seven-in-10 transplant operations in Australia are successful after a five-year period due to rejection complications. (Reporting by Rob Taylor; Editing by Alex Richardson)

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