Matching A Donor
Matching a stem cell donor is typically a concern associated with allogeneic transplants, since these involve stem cell sources donated from either a family member, unrelated individual, or a cord blood unit from a cord blood bank.
In the case of autologous or syngeneic stem cell transplants, an HLA match is guaranteed since the stem cell source comes from either the patient or the patient's identical twin who carries identical genes.
HLA matching is a key factor when it comes to stem cell transplants, whether cord blood or bone marrow transplants. Stem cell donor and HLA matching are integral to improving the success of a transplant, to promoting engraftment or the growth of new cells, and to preventing transplant rejection, also known as GVHD.
HLA Matching
Human Leukocyte Antigens (HLA) is a type of protein that is found on the outer surfaces of cells. HLA protein is made up of six antigens, molecules that stimulate an immune response and help your body identify which cells belong to your body and which do not.
There are three antigen groups that are considered important when it comes to matching for a stem cell transplant. These are HLA-A, HLA-B, and HLA-DR. Since each of these groups is made up of two antigens, one inherited from each parent, doctors must match a total of six antigens. This makes it more likely for a sibling to be a potential stem cell donor, with a 25% chance of inheriting the same antigens from the parents.
The benefits of cord blood transplants over a bone marrow transplant when it comes to HLA matching lies in the fact that cord blood is more flexible in cases of an incomplete match. This is because umbilical cord blood contains immature stem cells that are less likely to attack a patient's immune system. Therefore, a 4/6 HLA match may be acceptable for stem cell transplants, while a minimum of a 5/6 HLA match is required for a bone marrow transplant.
Finding A Stem Cell Donor
It is the responsibility of a transplant center to find a stem cell donor for a patient, and not the individual. A doctor may perform a preliminary search for a donor without obliging the patient to necessarily undergo a stem cell transplant.
A preliminary search can simply provide a glance at the potential cord blood units that may be suited to a given patient based on the individual's HLA tissue type. This search is completely free of charge, and further analysis would be necessary should a stem cell transplant treatment be chosen over other options.
If a donor match cannot be found, your doctor may recommend stem cell transplant alternatives. While a transplant center searches for a donor match, a patient will continue to receive treatment from their primary doctor, who will be in contact with the transplant center and notify them of any changes to the patient's health.
It can take as little as two weeks to find a cord blood unit match, while several months may be required to locate a bone marrow donor. Since there is a higher chance of matching HLA antigens within families, it may be recommended that family members, and particularly siblings, be tested as potential donors. In addition, a patient's parents or children may be tested to confirm HLA typing of the patient.
Other Factors
Many stem cell transplant centers will look at more than the crucial six HLA types of antigens in order to a select a donor and insure an ideal match. For instance, a transplant center might be looking for a 10 of 10 match, in which case other HLA antigens will be analyzed in addition to the three most important types (LA- A, -B, and -DRB1).
If more than one suitable match is found, a doctor may consider other factors such as the donor's age, sex, blood type, size, the number of times a female donor has been pregnant, and whether or not a donor tests positive for cytomegalovirus (a genus of the Herpes virus).
A donor that is under the age of 45 may be a more suitable choice since a younger donor may result in better transplant outcomes. The number of blood-forming cells in a unit is also a key factor when it comes to matching. When it comes to cord blood transplants, the number of stem cells found in a cord blood unit must be measured and selected in accordance to the patient's weight.
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