"This is an unrelated donor transplant to a first child and then taking the marrow second hand and giving it to somebody else. That's never been done before," Dr Paul Veys said.
Dr Veys, a bone marrow transplant specialist at Great Ormond Street Children's Hospital in London, said both Owen Vincent and his brother Niall, from Gloucestershire, suffered from a rare genetic disease called Severe Combined Immune Deficiency syndrome (SCID). They were born with no immune system and were unable to fight infections.
Most babies born with SCID had to spend their lives in plastic bubbles. Many died in their first year of life until scientists developed a technique to used bone marrow transplants to treat the disorder.
Owen received the partially matched bone marrow from an unrelated donor three years ago. After gruelling chemotherapy and long periods of isolation doctors said it was a success.
When Niall was born with the same disease this summer, Dr Veys and his colleagues decided to use Owen's bone marrow for the transplant.
Within 24 hours of his birth they transplanted Owen's bone marrow into Niall without using chemotherapy, which is usually essential so the transplanted bone marrow is not rejected.
"The thinking was that because the transplant bone marrow had spent three years in Owen it would now be tolerated by him and his brother," Dr Veys said.