2. Preliminary Blood and Tissue Compatibility
The first step is to determine your blood type and tissue type. Testing is done by drawing a blood sample. If you are a donor volunteer, these tests are part of the registration process. You may have to pay for the testing, which ranges from $50 to $100. The results are input into the marrow registry and used to make a preliminary match with compatible recipients.
Blood Type. The first test determines your blood type. There are four blood types designated by the presence or absence of two antigens—the A antigen and the B antigen. Blood type A means you have the A antigen. Type B means you have the B antigen. Type AB means you have both antigens. Type O means you have neither antigen.
You must have a blood type compatible with the recipient or you will not be able to donate. Here is who can donate to whom:
Type A can donate to types A and AB.
Type B can donate to types B and AB.
Type AB can donate to type AB.
Type O can donate to types A, B, AB, and O.
Tissue Type. The second test run on the blood sample identifies your human leukocyte antigens (HLA). There are many different kinds of antigens, but there are three categories assessed for marrow donation, designated HLA-A, HLA-B, and HLA-DRB1. You inherit one set of these three antigens from each parent giving you a total of six HLAs.
A similar test is run on a blood sample from the recipient, and the antigens are compared. You might hear of a "six-of-six" match (all donor and recipient antigens match), or a "half match" (three of the six antigens are the same), or a "zero match" (none of the antigens matches). In the case of bone marrow donation, tissue typing is critical, so a perfect "six-of-six" match is required.
3. Detailed Blood and Tissue Compatibility
Once a potential recipient has been matched on a preliminary basis, more detailed compatibility testing is done. Again, a blood sample is drawn from you and tested.
DNA Testing. One test that's done is DNA testing. This testing is similar to the HLA testing but is more detailed. It uses sophisticated laboratory tests to determine the DNA of your antigens and compares the DNA code to those of the recipient.
Crossmatching. Crossmatching is a further testing of antigen compatibility. In this test, white blood cells from you are mixed with blood from the recipient. If the white blood cells are attacked and die, then the crossmatch is "positive," which is a negative as far as your ability to donate. It means the recipient is "sensitized" to you—the recipient has antibodies to some of your antigens—so the recipient's immune system would turn on the donated marrow and destroy it. If the crossmatch is negative, you are compatible with the recipient.
Note: The blood and tissue typing is rigorous because compatibility between the donor and recipient is essential to a successful transplant. As a consequence, many potential donors are ruled out. The odds of two individuals matching range from 1 in 20,000 to 1 in 50,000 depending on such factors as ethnic background. This is the primary reason why unrelated donation is so important.