Graft versus host disease, or GVHD, is a problem that may happen after a stem cell or bone marrow transplant.

In a stem cell transplant, high doses of chemotherapy and sometimes radiotherapy are used to destroy all of your bone marrow. Stem cells (the graft) from another person (a donor) are then given to you. The new stem cells travel to the marrow and begin to grow there and produce new blood cells (engraftment). These new blood cells include the white cells that form the immune system. This means that you (the host) will have the immune system of the person who donated the stem cells.

The immune system normally works to spot 'foreign' or different cells in the body, such as bacteria or viruses, and attack them. It is what helps protect us from infection. Sometimes your new immune system recognises you as different from the donor who gave the stem cells. As a result the donor's white cells attack you (the host). This is GVHD.

If this happens in the first three months after the transplant it is called 'acute' GVHD. The parts of the body that are most likely to be affected are the liver (causing jaundice or liver damage), the skin (causing rashes or blistering) and the gut (causing diarrhoea and tummy pain). Often small samples (biopsies) need to be taken from these organs to confirm the diagnosis.

Sometimes GVHD develops slowly, several months after the transplant. This is called chronic GVHD and can affect lots of organs in the body. Problems this might cause include a dry mouth and eyes or changes in the skin and stiff joints. It may also cause breathlessness and blood tests may show the liver is affected. Usually biopsies need to be taken from the affected areas.

Doctors divide GVHD into groups based on how much the organs are affected. This gives an idea of whether it needs to be treated. Treatment of GVHD, is with drugs that damp down or switch off the new immune system to stop it attacking you. Drugs commonly used include cyclosporin and steroids.

There is a lot of variation in how GVHD affects people. Some people have no GVHD, while in some people it is very serious and even life-threatening. This difference seems to be related to how similar the donor is to the patient. Transplants between identical twins cause very little GVHD, while transplants between unrelated people have much more GVHD. Despite tissue typing and careful matching of the donor and patient before a transplant, GVHD still happens. This is because there are a lot of differences apart from the tissue type that we don't know about yet but that can trigger GVHD.

GVHD is not entirely a bad thing. People who have some GVHD have a lower risk of the cancer coming back (relapsing). It seems that GVHD is sign that the immune system is likely to be more active in attacking any cancer cells that may still remain in the body. This is called the ‘Graft versus leukaemia effect’.