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When the blood type is only a near match, cord blood stem cells are unique and in turn found to be successful in
transplant cases—as opposed to stem cells found in bone marrow—which require an exact match from donor to recipient.
During the past decade, over 2000 cord blood transplants have been undertaken in both children and adults .
Initially the cases were those with no compatible bone marrow donor and the patients would have died without any cord blood
transplant. It was found that there are survivors, reduced complications such as graft versus host disease (GVHD) but delayed
engraftment. Success rate is related to the dose of cells transplanted in relation to the weight of the patient and also to
the degree of (human leukocyte antigen) HLA mismatch. In other words, HLA matching is still important but not as critical as
for a bone marrow donor. Cord Blood stem cells do not need such rigorous matching and if there is a large number of banked
cord blood, over 90% of patients would be able to find a compatible cord blood for transplant.
HLA Matching
Human leukocyte antigens (HLA) are proteins on most of the body's cells. These antigens help identify a
person's tissue type. Your immune system uses HLA antigens to recognize which cells belong in your body and
which do not. The HLA proteins (two A antigens, two B antigens and two DR antigens) are important in matching
patients and donors for a blood stem cell transplant. When a transplant center looks at the match level, it is
looking at how alike the tissues of the patient and the donor are to each other.
The difficulty in finding a suitable donor lies in the fact that the donor's and patient's "tissue type" must
closely match in order for the transplant to be successful. Genetic markers on the surface of white blood cells
called HLA-antigens define a person's tissue type. Since these genetic markers are inherited, siblings are much more
likely to have similar HLA-antigens than unrelated persons.
There's a 30-35 percent chance that a patient's sibling will be a suitable donor. (i.e. a one-in-four chance of any
one sibling being a match, with increased odds in larger families). If a donor must be located in the general population,
the chances of finding a match range from one in 1,000 to one in several million, depending on the frequency of the
patient's tissue type in the general population.
To minimize the risk of graft rejection and graft-versus- host disease, a donor whose HLA type matches that of the
patient is best. The optimal donor is often an identical twin. Not only will the twin have inherited from the father
and mother the same antigens at the major loci (HLA-A,-B, and -DR) as the patient, but the antigens at tissue antigen
sites other than HLA sites that are more difficult to detect or whose role in transplantation is unclear will also match.
The risk of either graft-rejection or severe GVHD in BMTs using marrow from an identical twin is eliminated. In other cases,
the best bone marrow donor will be a sibling who is not an identical twin, but whose HLA-A, -B, and -DR antigens
match those of the patient.
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