Human leukocyte antigen (HLA) typing is a method to determine how closely the tissues of one person match the tissues of another person. Human leukocyte antigens are proteins you inherit from your parents. Together, your HLA proteins, or markers, make up your HLA type. We currently know of more than 2,500 different HLA markers.
It is important in bone marrow transplants to know how closely the HLA type of the transplant recipient matches the HLA type of the donor. We can determine your HLA type before you have a transplant by taking a sample of your blood or another body tissue. The same process can be used to determine the HLA type of anyone who may donate to you.
What Is an HLA Match?
The HLA match is the number of HLA markers that any two people have in common. HLA matching is usually based on either eight or 10 HLA markers. The more markers two people share, the better the match. When two individuals share the same HLA markers, they are said to be a good match. That is, their immune systems will not see each other as foreign and are less likely to attack each other.
The most likely place to find an HLA match between two people is among siblings who have the same mother and father. If two siblings inherit the very same HLA markers from both parents, they are said to be an HLA-identical match.
- You have a 25-percent (1-in-4) chance of being an HLA-identical match to your sibling—that is, inheriting the very same HLA markers as your sibling.
- You have a 50-percent chance of inheriting half of the same HLA markers as your sibling. This is called a haploidentical match.
- You have a 25-percent chance of inheriting none of the same HLA markers as your sibling.
Two unrelated people can just happen to be a good HLA match, too. Although it is less likely, it is possible that you could have some of the same HLA markers as someone you don’t even know.
Finding the Best Match
When a doctor decides that a bone marrow transplant is the best treatment for a patient, the patient, all of his or her siblings, if they agree, and sometimes their parents will have samples collected for HLA typing. If one of the family members is an HLA-identical match, the lab will do further testing to be absolutely sure that they are the best match possible.
If none of the siblings is a good HLA match, the doctor will sometimes ask to have additional family members tested. Since your HLA type is inherited from your parents and passed on to your children, your parents and children have the next best chance among your relatives of being closely HLA matched with you.
If there are no close HLA matches within the family, the health care team will contact the National Marrow Donor Program to search international registries for an unrelated volunteer donor with the same HLA markers as the patient.
What Are HLA Antibodies?
Before transplants, patients are tested for HLA antibodies. HLA antibodies are proteins that may be present in the patient’s blood and could interfere with the success of the transplant. If the donor is not an absolutely perfect match, the patient’s HLA antibodies may attack the donated cells and make the patient’s body reject the transplant.
Some people do not have HLA antibodies, while others do. The reasons are not clearly understood, but people are most likely to develop HLA antibodies from pregnancies, prior blood or platelet transfusions, or organ transplants.
The HLA antibody level is referred to as the PRA (panel reactive antibody). The PRA tells us the percentage of the general population that the patient has HLA antibodies to. This can give us a good idea about how easy or difficult it will be to find a compatible platelet donor for this patient. In addition to measuring the patient’s PRA, we can also test whether a patient has antibodies to a particular HLA molecule. Some patients have antibodies to one or two HLA molecules, while others have antibodies to many HLA molecules and, therefore, have a higher PRA.
Request HLA Typing at SCCA
At Seattle Cancer Care Alliance (SCCA), we typically receive requests for HLA typing from treating doctors—for example, your primary oncologist. While we can certainly discuss HLA typing with you directly, we recommend that your doctor be involved to work with us.
When we receive your request, the SCCA intake department will work with your local doctor to determine three things:
- What is the clinical urgency, based on your condition?
- What is your treatment plan?
- Are you considering searching for an alternate donor if there is no family match?
After determining the items above, we will contact you to explain the HLA typing process and collect information about your insurance and your family member who is willing to donate. We will contact your insurance carrier for financial clearance, and we will communicate with you about any financial implications of HLA typing. If we need to discuss altering the HLA-typing strategy based on any of the information collected, one of our doctors will contact your local doctor or you.
Our Clinical Immunogenetics Laboratory will send instructions to your local doctor about how to collect and send specimens for HLA typing.
If you want to discuss HLA typing, call our intake department, Monday through Friday, 8:30 a.m. to 5 p.m., at (800) 804-8824 or (206) 606-SCCA (7222).