Injecting healthy blood cells into transplant patients with cancer can improve survival, the first clinical study of the treatment suggests.
The treatment was pioneered on Mohammed, now nine and healthy
Up to 10% of the 3,000 people who undergo a transplant each year are at risk of a cancer called post-transplant lymphoproliferative disease (PTLD).
This is thought to be triggered by a virus which strikes easily because their immune systems are suppressed.
But the study in the journal Blood said donated white cells can kill the virus.
All the blood donors used had been exposed to the Epstein-Barr virus (EBV), carried by 90% of the population.
In most people, the virus does not cause cancer to develop.
But transplant patients have weak immune systems because of the drugs administered to stop their bodies rejecting the organs, and this is thought to make them more susceptible.
Researchers at Edinburgh University extracted "killer-T" cells - a white blood cell which attacks virus infected cells - and injected them into 33 transplant patients with PTLD.
'Normal little boy'
All the patients had a poor prognosis, having failed to respond to conventional therapy such as reducing the dose of immunosuppressants, chemotherapy and radiotherapy.
Over the nearly eight-year trial, 14 patients out of 33 involved achieved complete remission from their cancer, and tumours in three of the patients shrank by 50% or more.
But 16 patients had no response after six months, and five died before completing the treatment.
The approach was pioneered in 2001 on a four-year-old boy called Mohammed from West Yorkshire, whose condition was so dire doctors said they had little choice but to opt for an untested treatment.
Now nine, he remains cancer-free and leads a normal life.
His father Kahlid said: "Without this treatment, Mohammed wouldn't be here today. We are delighted to hear that this research has been successful and will now be able to benefit other families like us."
Child transplant patients are thought to be the most at risk from the disease as they are less likely to have been exposed to EBV.
The T-cells from those who have been exposed automatically recognise the disease in their new host and kill PTLD cells loaded with the virus.
Cancer Research UK, which funded the study, believe that if further trials prove successful, a single bank of T-cells could serve as a global source of immune cells to treat the condition.
A spokesman for UK Transplant, the NHS organisation that matches and allocates donated organs said: "These results are very encouraging.
"Organ transplants save thousands of lives every year and any treatment that helps patients continue to enjoy a healthy life post-transplant is to be welcomed."