BBC Science Correspondent James Wilkinson reports

Three-year-old Owen Vincent was given the chance to save the life of his baby brother, Niall.

Owen, who is featured in the BBC programme QED, was born with severe combined immune deficiency syndrome (SCID).

At first, his mother Anne did not suspect anything was wrong with him. But when she stopped breast-feeding him at four months, he caught a cold which never seemed to dry up.

Owen then became very ill and was found to have pneumonia in both lungs.

SCID babies have no white T blood cells

T cells

He was diagnosed as suffering from SCID, a syndrome which means he has no white T blood cells which provide natural resistance against infection. Without these, infections spread and can be fatal.

After being diagnosed, Owen was put in isolation in a bubble full of filtered air to protect him from infection while he waited for a suitable bone marrow match.

The only treatment for SCID children is a bone marrow transplant to jump start their immune system, but the operation is dangerous.

Owen in his hospital bubble

The first SCID baby transplant operation, performed around 20 years ago, failed and the baby died.

Doctors found only one suitable donor for Owen, but the match was not good.

However, the operation was a success and Owen is now perfectly normal. His mother said: "He loves kissing and cuddling and it is hard to think of the time when all he could do was touch his friend's hands through glass."

Life-saving

However, Owen's ordeal was not over. When Anne became pregnant with Niall, he was doctors' choice for a life-saving operation.

Bone marrow transplants are dangerous

Because doctors knew there was a possibility that Niall would also have SCID, they were prepared.

As soon as he was born, they did a blood test which showed he had no T cells. The tests also showed Owen was the perfect match for Niall.

Owen was on hand to undergo a bone marrow transplant operation. His white cells were transplanted into Niall.

Doctors said this was safer than using Owen's donor as he was a closer match and his body had successfully transformed the cells he received.

Foreign bodies

The transplant operation, carried out at University College Hospital in London, took just 45 minutes.

Unlike Owen, Niall did not need chemotherapy after the operation. His blood was tested a few days later and T cells were found in his body.

Niall is now a healthy boy

There was still, however, a risk that the foreign T cells could turn on Niall.

A few days later, Niall was checked for rashes to see if the cells had started to attack his body.

They hadn't and two and a half weeks after his birth, Niall was sent home.

Doctors say he appears to have successfully overcome SCID with little suffering.

The trip home was an emotional one for Anne. Her mother had lost a son and three of her grandmother's sons had died young.

It is now thought they all had SCID. "I felt the family was cursed and now I have two sons I feel the curse is finally over," said Anne.

No symptoms

Many SCID babies are not diagnosed soon enough to save them.

Ben Brown: died in June after late diagnosis of SCID

Elaine Brown's son Ben died in June. She had two healthy daughters and Ben showed no symptoms when he was born.

Then he began to get infection after infection. At four and a half months, he was coughing like an old man.

Elaine went to her GP several times. She said she knew something was wrong, but was told not to worry.

At eight months, x-rays showed Ben had a collapsed lung and pneumonia.

He tested positive for SCID and was put in a sterile bubble in hospital while a bone marrow donor was sought.

However, he died before one could be found. Elaine believes he could have been saved if he had been diagnosed earlier.