From this link: http://www.ibmtindy.com/faq/hla-typing.htm
Special blood tests, called Human Leukocyte Antigen (HLA) typing, determine whether a patient has a suitable donor for stem cell transplant. These tests are performed on several tubes of blood drawn from the arm The matching for stem cell transplant is much more complicated than matching for red cell blood types. HLA typing is increasingly done using DNA techniques and can take several days to complete. (For information on the HLA Lab at St. Francis please refer to their on-line information at http://www.stfrancishospitals.org/hla/).
The most likely place to find a bone marrow match is within the patient's own family. Siblings (brothers or sisters) will more likely match than other relatives, such as parents, children, or cousins. All humans inherit 1/2 of their entire genetic make-up, and thus of their HLA-type, from their mother and the other 1/2 from their father. For each full sibling, a patient has a one in four (25%) chance of a full match. If you have full siblings, the chances of having a completely matched donor are:
(The chance of having a donor is 1-(3/4)n, where n is the number of siblings).
With the current size of the average American family, only about 30% of patients will be found to have an acceptable HLA-matched donor within their family. Therefore, search for an unrelated donor may be necessary.
What About Unrelated Donors?
The National Marrow Donor Program (NMDP) is a federally supported organization that assists in locating unrelated donors. Currently, the NMDP has computer access to over 2 million volunteer donors from throughout the United States and other countries. How easily an unrelated bone marrow donor can be found, is dependent on the patient's HLA type. The more unique, or unusual, the HLA type, the more difficult it will be to find a sufficiently matched unrelated donor. Patients from mixed ethnic background may have very uncommon HLA- types. In addition, although the NMDP has made an enormous effort to recruit minority donors, the chances for African-Americans and Asian- Americans of finding a fully matched unrelated donor are still smaller than for Caucasian patients. Even in cases where the patient's HLA type is quite common, it can take a minimum of 4 to 6 weeks to obtain the donor's marrow. For more information about unrelated stem cell donors, and how you can become a donor, you may directly contact the NMDP (http://www.marrow.org/).
What If No Unrelated Donor Can Be Found?
In some instances, the HLA type of the patient may be so unique that finding a matched unrelated donor is impossible. If a patient cannot find a fully matched related or unrelated donor, it may be necessary to look at other options, such as using a patient's own bone marrow, a partially matched family donor or umbilical cord blood. The results of partially matched related transplants have improved considerably in the last few years. Such transplants are now feasible, but the risks are still higher than for HLA-identical sibling transplants or transplants from matched unrelated donors.