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, say doctors.

©Paul Miller/AAP
Demi-Lee Brennan with Dr Stephen Alexander (left) and Dr Stuart Dorney (right) at Westmead Hospital in Sydney - the teenager no longer needs anti-rejection drugs

A Sydney-based team describe the case in the latest issue of The New England Journal of Medicine.

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

Her 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.

The stem cells from the donated liver then invaded Brennan's body's bone marrow and triggered her entire immune system to switch to that of the donor's, meaning the teen no longer needed anti-rejection drugs.

Doctors say they have no explanation for Brennan's recovery, and describe her as a "one-in-six-billion miracle".

For Brennan herself, it's hard to believe.

"It's like my second chance at life," Brennan says, recounting how her body achieved what doctors said was the holy grail of transplant surgery.

Paediatric hepatologist Dr Michael Stormon, of The Childrens' Hospital at Westmead, who is one of the authors of the journal report says there was no precedent for this having happened.

"So we were sort of flying by the seat of our pants," he says.

Dr Stuart Dorney, the hospital's former transplant unit head, says 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," says 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.