By JENNY HOPE

Last updated at 11:48 26 January 2008

A teenager has become the first transplant patient to switch blood groups - at estimated odds of six billion to one.

Demi-Lee Brennan had O-negative blood before receiving a replacement liver six years ago.

However, she is now O-positive after her body adopted the immune system of the organ's donor.

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The switch means the 15-year-old no longer needs immunosuppressant drugs which are used to stop the body rejecting a new organ.

The drugs are usually taken for life and only a small number of transplant patients have been able to come off them. None is thought to have done so by jumping blood groups.

Experts say that studying what happened to the Australian teenager may open the way to beating organ rejection - the holy grail of transplant medicine.

Richard Thompson, a paediatric hepatology expert at King's College London, said Demi-Lee's case appeared to be unique.

He said: "It's a dramatic, remarkable case and she's been incredibly lucky.

"A small number of people can stop taking anti-rejection drugs altogether after a transplant because they develop a tolerance to the donor organ but this has not been achieved before by a change in blood group.

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"It can be extremely dangerous to take patients off immunosupressants to see what happens.

"It can result in rejection and the patient then needs a new organ."

Doctors try to give patients organs from donors of the same blood group.

But Demi-Lee was so ill that medics took a chance when a mismatched liver became available at Westmead Childrens' Hospital in Sydney.

Ten months later she developed lifethreatening anaemia. Tests showed stem cells from the donor were taking over her bone marrow, the site where blood cells are produced.

In desperation, doctors withdrew Demi-Lee's anti-rejection therapy - allowing the donor cells to overwhelm her own. It worked and within a few

months her blood type had switched completely to that of the donor.

Four years on she remains healthy - as does her liver. "It's like my second chance at life," she says. "It's kind of hard to believe."

Doctors who treated Demi-Lee say they cannot fully explain her recovery, which is detailed in the latest edition of The New England Journal of Medicine.

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

Stuart Dorney, the hospital's former transplant unit chief, said: "We now need to go back over everything that happened to Demi-Lee and see why, and if it can be replicated. We think because we used a young person's liver and Demi-Lee had low white blood cells, that could have been a reason."

Meanwhile, U.S. scientists are pioneering a way of re-engineering the immune system in transplant patients to accept mismatched organs without the need for anti-rejection therapy.

They are using bone marrow taken from the donors at the same time as the organ to persuade the body to accept foreign material as its own.

Dr Thompson said the results from the trial of kidney and liver transplant patients offered hope because six had remained healthy for up to five years without anti-rejection drugs.

"The ideal is to reprogramme the recipient's own bone marrow to accept a mismatched donor organ but we're a long way off achieving it," he said.