If you have had stem cells from a donor, even a brother or sister, there is a possibility that the new cells (the graft) will react against your tissues (the host). This reaction is called graft versus host disease (GvHD). This happens when the donor’s immune system attacks the patient’s body. You will be prescribed medicines to prevent GvHD happening, so it is usually mild. However, in some people it can become very severe and even life-threatening. It mainly affects the skin, the gut (stomach and bowel) and the liver. The reaction can occur up to six months after your transplant.

Occasionally the donor’s bone marrow may be treated (purged) with antibodies to remove a type of white blood cell (T-lymphocyte) which is thought to cause the GvHD. Removing the T-lymphocytes is most often done if a severe graft versus host reaction is possible – for example, if bone marrow from an unrelated donor is used.

If GvHD occurs, it does not mean that the transplant has failed. It may even be helpful, as some of the cells involved in the reaction may also attack any cancer cells in the body that have survived the high-dose treatment.