In a stem cell transplant, high doses of chemotherapy and sometimes radiotherapy are used to destroy all of your bone marrow. Stem cells from another person, a donor, are then given to you, similar to having a blood transfusion. The stem cells set up home in the marrow spaces and grow to produce all the blood cells you need. This includes all the white cells that form the immune system - meaning you will have the immune system of the person who donated the stem cells.

The immune system normally works to spot different cells in the body, such as bacteria or viruses, and attack them. It's what helps protect us from infection.

Your new immune system will work in the same way and recognise that the leukaemia is different and attack it. This is the ‘graft versus leukaemia effect’ [GvL]. The donor stem cells may also recognise that you are different and attack you and cause ‘graft versus host disease’ [GvHD].

Doctors now know that the graft versus leukaemia effect and graft versus host disease are linked. So, people with graft versus host disease have a lower risk of the leukaemia coming back. We also now know that patients with a small amount of relapsed disease can be put back into remission by having a top up of donor white cells. This tips the balance in favour of the donor's immune system attacking the leukaemia.

The graft versus leukaemia effect varies for different diseases. It is strongest for chronic myeloid leukaemia, then acute myeloid leukaemia and is least for acute lymphoblastic leukaemia.

We don't understand exactly how graft versus leukaemia works. But, it would be ideal if we could get the benefits of graft versus leukaemia without the risks of graft versus host disease.