The first step in finding a donor is for you to have a blood test to find your tissue type. Your tissue type is the combination of proteins on the surface of your cells called human leukocyte antigen (HLA) markers. Your immune system uses your HLA markers to recognise your own normal body cells.
Doctors use your HLA markers to match you with a donor. When they know your tissue type, they compare it with close relatives or with donors on a registry. They look to see if the tissue type is the same as yours (a match). Doctors look for between 8 to 10 markers that match with yours. Occasionally all 10 will match. Having the best possible match means less risk of:
- your body rejecting the new stem cells (graft rejection)
- the new immune cells reacting against your other body cells (graft versus host disease).
The best match is usually a brother or sister (your siblings). If you don’t have close family, or they are not a match, your doctors look for a matched donor through a volunteer registry.
Your donor does not need to have the same blood group as you. After the transplant, your blood group changes to their blood group.
If doctors can’t find a well-matched donor, there are other possible approaches. They may accept small differences to improve the chances of finding a donor. This is known as a mismatched transplant. This could be someone in your family or someone on a donor registry. A more recent option is a relative whose tissue is a half-match to yours (haploidentical). Another possible option could be using a cord blood transplant.
Stem cells from a matched related donor
Brothers or sisters (siblings) each have a 25% chance of having the same tissue type as you and being a match. Our tissue types are a combination of both parents. Not every combination will be the same. So not every sibling will be a close enough match to be a donor.
Parents, half-brothers, and half-sisters are not usually a good match.
Occasionally a sibling has one HLA marker that is different but can still be used as a donor. This is known as a mismatched transplant.
A different team from the one looking after you will contact your family to ask if they are willing to be tested. Your siblings can be tested even if they live abroad.
Stem cells from a volunteer donor
If you do not have a close match in your family your doctors will look for a volunteer donor who is not related to you. There are donor registries in the UK, such as Anthony Nolan, which have lists of possible volunteer unrelated donors.
If there is not a suitable UK donor for you, there are registries in other countries that your doctor can search. It usually takes longer to prepare an international donor than a UK donor. Occasionally no suitable donor can be found.
Half-matched donors (50% match)
When there is no matched donor available, your specialist may advise using stem cells from a close relative whose tissue type is a 50% match to yours. A close relative is a brother or sister, child, or parent. Almost everyone will have at least one possible donor. Doctors call this type of transplant a haploidentical transplant.
As your donor is related, it can usually be done quickly. But it is still quite a new approach and will not be suitable for everyone.
Because it is only a 50% match, there is more risk of complications, such as GVHD, graft rejection and slow recovery of the immune system. But doctors are finding different ways to deal with these problems.
Stem cells from cord blood
Another source of stem cells is blood from an umbilical cord. This is the cord that connects a baby to its mother during pregnancy. Cord blood can be donated after birth and frozen. It is stored in a cord blood bank and used for people needing transplants. There is no risk to the mother or baby and all cord blood is tested to make sure it is safe to use.
It may be possible to use cord blood for people who do not have a suitable donor. With cord blood, you do not need as close a match as with an adult donor. You can also get it quickly.
A person’s weight can be a factor when doctors are deciding whether a cord blood transplant is suitable. People who weigh more need more stem cells for a transplant to be successful. It may not always be possible to get enough stem cells from cord blood.
Sometimes doctors can use cord blood from two different cords in one transplant if they are both a close match. This is called a double cord blood transplant.