A liver-transplant patient from The Children's Hospital, at Westmead, is being hailed as a "one-in-six-billion miracle girl" after spontaneously taking on the immune system of her organ donor.
The breakthrough is a world-first in transplantation medicine.
Typically, patients who receive liver transplants need life-long immunosuppressive medication, which has significant side effects including infections and toxic effects on some organs.
The patient is Demi-Lee Brennan, who lives in Kiama. When she was nine-years-old, she was diagnosed with severe liver failure, or acute fulminant hepatitis. She required a life-saving liver transplant, which was performed at the hospital five years ago.
The donor was a 12-year-old boy who had died from a brain injury.
As in all transplant patients, Demi-Lee received immunosuppression therapy medication to prevent the patient's body from rejecting the new organ.
But Demi-Lee became seriously ill nine months after her transplant after a small bowel obstruction developed.
After routine blood tests, doctors were shocked to discover that Demi-Lee's blood group had changed to that of her donor from O, RhD-negative to O, RhD-positive.
Even more shocking was the revelation that stem cells from the donor liver had penetrated her bone marrow, resulting in a bone marrow transplant.
Demi-Lee's immunue system had almost been totally replaced by that of the donor, the Children's Hospital said.
As a result, the immunosuppressant medication was stopped 14-months after the transplantation and Demi-Lee's body took on the new immune system.
Stuart Dorney, the director of Clinical Governance at the Children's Hospital, was Demi-Lee's treating doctor. Dr Dorney said that out of 150 patients who received liver transplants at the hospital, Demi-Lee was the only person to have stopped using the anti-rejection medication.
"Liver transplants are fairly routine, but what happened after wasn't," Dr Dorney said.
"To have this happen is the most amazing thing."
After the immunosuppressive therapy ceased, the loss of anti-body responses to measles and mumps occurred but were restored after re-vaccination.
Demi-Lee, now 15, has not taken any immunosuppressive therapy for four years.
Demi-Lee's liver function tests remain normal and she has had no symptoms to suggest graft-versus-host disease, where immune cells of the donated organ attack the body of the transplant patient.
"I don't really understand what happened and only when I look back at photos I remember that it was me," Demi-Lee said.
"I'm really thankful and hope so many other people can [donate organs] too.
"I feel totally thankful about my second chance at life."
Demi-Lee said she hoped to meet her donor's parents when she turned 18. She has been writing letters to the family but they do not know any details of Demi-Lee's transplant.
The medical breakthrough has prompted doctors at Westmead to further research why this occurred, hoping to use their findings to improve treatments and outcomes for other transplant patients.