To reduce the risk of your body rejecting the donated stem cells (graft rejection), the donor’s tissue type has to be closely matched to yours. The matching process involves a blood test, and is done by looking for specific proteins known as markers on the surface of the cells. The markers are called human leukocyte antigens (HLAs). Once the HLA type of your bone marrow has been found, other people can be tested to see whether their bone marrow and stem cells are the same type as yours. The closeness of a match needed for a good result depends on which particular tissue markers are the same as yours. For some markers, doctors may decide to accept small differences, to improve the chances of finding an acceptable donor. In this situation, the transplant is known as a mismatched transplant.

Donors need to be in good health. They will be given a thorough medical check-up to make sure that there will be no risk to their own health from the procedure. The donor will have to go to the hospital for a couple of outpatient visits for these checks. They then have either their bone marrow or stem cells collected at the hospital.

Usually brothers and sisters (your siblings) are tested first, as they are likely to have the best match. Because of the way that your siblings inherit the 'HLA markers' from your parents, some will be more likely to be a good match, others will be less likely. If one of your siblings is completely matched to you, they are known as an HLA identical donor. The match is unlikely to be perfect unless you have an identical twin. A transplant using bone marrow or stem cells from an identical twin is called a syngeneic transplant. Parents, half-brothers and half-sisters will not usually be a good match, although they may be tested if you have no other siblings, or if your doctors have not been able to find a complete match for you.

If you have no suitable relatives it may be possible to find a matched unrelated donor (known as MUD). The National Blood Service (England and Wales), the National Blood Transfusion Service (Scotland) and some charities have lists of volunteer unrelated donors. Your doctor will have access to these and can search for a close match for you. Remember, however, that it may be very difficult and time-consuming to find a good match. Overall, only about 1 in 10 searches will find a closely matched, unrelated donor.

People from ethnic groups often have difficulty finding a good match from the volunteer registries. This is because most people who register as potential bone marrow donors are from the white population, and tissue types rarely match across different ethnic groups.