The oncologist’s office or transplant center (if patient is already being seen there) will assist patients in making arrangement for HLA testing of the basic A, B, and DR antigens for themselves and their sibling(s). Special arrangements can be made for testing of siblings living out of area and/or out of country. These tests are often, but not always, covered by insurance plans, public or private. See the FAQ page for more information about insurance overage of HLA testing.
HLA typing is reported as a series of numbers. For example, an HLA type will appear on the report as A 3, 32; B 7, 37; DR 1, 15.
Role of Genetics
Each of the HLA proteins or antigens is inherited from one’s biological parents, and they tend to be inherited in a group called a haplotype. The six different antigens then are paired up in different arrangements, whose general pairing patterns are hereditary. Because the transmission of the haplotypes is genetic, matching of the patterns is most likely between biological siblings, and next among people of similar ancestral genetic / historical ethnic origins. Due to the odds of any particular combinations of haplotypes being inherited, there is about a 1 in 4, or 25% chance of finding match between a patient and a biological brother or sister.
What is a match?
A BMT “match” is made by matching up as closely as possible certain proteins called antigens, found on the surface of most cells, that regulate the ability of the immune system to recognize cells as “self” that belong in the body and to fight off those which are “foreign” which do not belong there. Antigens mount a reaction to “invader” cells by stimulating antibodies to fight them off. The antigens important to the BMT procedure are the HLA or Human Leukocyte Antigens. These found in especially high concentrations in white blood cells.
What HLA Testing Shows
The HLA system includes many different types or groups of HLA antigens that are identified by different letters of the alphabet. There are specific groupings of HLA antigens important to solid organ transplants, hematopoeitic cell transplants, and platelet transfusions. The principal groups of importance for transplants of all 3 stem cells sources: bone marrow, peripheral blood, or cord blood include:
As shown above, specific HLA antigens are identified by a letter for their group (A, B, or DR), and then by a number, in order to distinguish antigens from the same group. An example of this pattern of identification is: HLA-A1, B8, DR17 for one person; and HLA-A2, B7, DR15 for another person. There are patterns to the occurrence of, and to the frequency of occurrence, of specific HLA antigens among distinct ancestral-origin population groups of the world.
HLA antigens can also be separated into Class I and Class II molecules. Each class has a different structure and different function in the immune system. Class I molecules include the HLA-A, B, and C antigens and are found on nearly all cells in the body. Class II molecules include the HLA-DR, DP, and DQ antigens and are typically only found on cells of the immune system although all cells with a nucleus have the ability to express these molecules.
Selecting the Cell Source for BMT
After these test results are received, the treating physician can discuss what type of transplant is indicated: either one using the patient’s own cells (autologous transplant) or using cells from a donor. This will depend on the patient’s specific disease, and present medical condition. While in some cases autologous BMTs may be done routinely, many times the decision on type of transplant is based on whether or not the patient has a matching biological sibling donor available, or if a search will need to be made for an unrelated donor. The treatment plan will also include estimated timing for a transplant, and referral to a BMT center.