Human leukocyte antigen (HLA) typing is a method to determine how closely the tissues from one person match the tissues from another person. Human leukocyte antigens are protein molecules you inherit from your parents. Together, your HLA molecules make up your HLA type. We currently know of more than 2,500 different HLA molecules.
It is important in bone marrow transplants to know how closely the HLA type of the transplant patient matches the HLA type of the stem cell 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 bone marrow or stem cells to you.
The HLA “match” is the number of HLA molecules that any two people have in common. HLA matching is usually based on 10 HLA molecules. The more molecules two people share, the better the match. When two individuals share the same HLA molecules, 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 (that is, brothers and sisters who have the same mother and same father). If two siblings inherit the very same HLA molecules 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 with your sibling, or inheriting the same HLA molecules as your sibling.
- You have a 25-percent chance of inheriting none of the same HLA molecules as your sibling.
- You have a 50-percent chance of inheriting half of the same HLA molecules as your sibling.
Your chance of an identical match increases with more siblings.
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 molecules as someone you don’t even know.
When a doctor decides that a bone marrow transplant is the best treatment for a patient, the patient, all of his or her siblings 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, among your relatives, your parents and children have the next best chance of being closely HLA matched with you. If it happens that there are no close HLA matches within the patient’s family, an search can be initiated in order to find an unrelated volunteer donor with the same HLA molecules as the patient.
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 them. Also, the patient may need transfusions of platelets during the recovery period. HLA antibodies can interfere with platelet transfusions by killing the donated platelets before they have a chance to work.
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 an 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.
At Seattle Cancer Care Alliance (SCCA), we typically receive HLA typing requests 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 insurance and family member information. We will contact your insurance carrier for financial clearance and communicate financial issues to you to ensure you are aware of any financial implications from HLA typing. If we need to discuss altering the HLA typing strategy based on any of the information collected, our clinical coordinator doctor 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, from 8:30 a.m. to 5 p.m. (Pacific time), at (800) 804-8824 or (206) 288-SCCA (7222).