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, 15, is the first known transplant recipient to adopt the blood type and the immune system of her organ donor, to the amazement of the medical community and Brennan alike. "It's like my second chance at life," she said. "It's kind of hard to believe."

"It's like my second chance at life," Brennan told local media

Brennan's body changed blood group from O negative to O positive when she became ill while on drugs

Doctors

said they had no explanation for Brennan's recovery, detailed in the latest edition of The New England Journal of Medicine.