Allogeneic bone marrow transplants, by definition, rely on the availability of a healthy donor whose bone marrow is biologically compatible with yours—what doctors refer to as an “HLA match.” HLAs, or human leukocyte antigens, are protein “markers” found on the surface of white blood cells.
They serve as a kind of genetic fingerprint, enabling the body’s immune system to recognize the body’s own cells. HLA markers occur in pairs, with one part of each pair inherited from a mother and one from a father. Therefore, the search for a matched donor begins within your immediate family. There is a 25 percent (1 in 4) chance that any one brother or sister perfectly matches a sibling’s HLA type. The overall chances of having any sibling who is a match depend upon the number of siblings.
Potential donors will need to be tissue-typed, which involves taking a series of blood tests to determine if the HLA “fingerprints” match. If a brother or sister doesn’t match, parents are screened for HLA compatibility, followed by the extended family of aunts, uncles, cousins, etc.
If a sibling or family match is not available, your transplant center should have procedures for finding an unrelated donor through national bone marrow registries. They will search a database of donors to identify a potential match. On average, the chances of finding an unrelated donor with a similar ethnic background are 60 to 70%. If someone is found, the registry contacts the potential donor with instructions about how to proceed. The identity of the donor is always kept confidential for a period of time following your transplant, after which point you and/or the donor are free to contact one another if both give written consent.
As successful as national registries have been in helping identify unrelated donors, many people in need of a BMT/SCT are unable to find a matched donor. To help fill this unmet demand, alternative sources of stem cells are being explored. For example, blood harvested from the umbilical cord of newborn babies (at no risk or pain to them) is a rich source of stem cells, and the use of cord blood in the BMT/SCT setting is increasing.
Related donors who are not exact HLA matches may also present an alternative for BMT/SCT candidates with no matched relatives. Such “HLA mismatched” procedures need to be discussed with your doctor. Such donors have not previously been a viable option because of the high risk of Graft vs Host Disease (GVHD) and graft failure associated with such mismatches.
Recent advances have helped make this an option in some cases.
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