HLA Matching: Finding the Best Donor or Cord Blood Unit

Human leukocyte antigen (HLA) typing is used to match patients and donors for bone marrow or cord blood transplants (also called BMT). HLA are proteins — or markers — found on most cells in your body. Your immune system uses these markers to recognize which cells belong in your body and which do not.

A close match between your HLA markers and your donor's can reduce the risk that your immune cells will attack your donor's cells or that your donor's immune cells will attack your body after the transplant. If you need an allogeneic transplant (which uses cells from a family member, unrelated donor or cord blood unit), your doctor will take a blood sample to test for your HLA type.

HLA matching basics

A well-matched donor is important to the success of your transplant. You inherit half of your HLA markers from your mother and half from your father, so each brother and sister who has the same parents as you has a 25% chance of matching you. It is unlikely that extended family members will match you. However, your parents and/or children may also be tested to confirm your HLA typing and to make sure no possible donors are overlooked.

About 70% of patients who need a transplant do not have a suitable donor in their family. If you do not have a donor in your family, your doctor can contact National Marrow Donor Program^® (NMDP) to search our Be The Match Registry^® for an unrelated donor or cord blood unit. The registry lists more than 8 million potential donors and more than 100,000 cord blood units. Patients searching the Be The Match Registry also have access to an additional 5 million donors through NMDP's agreements with international cooperative registries. 

Role of HLA matching

HLA matching is important because a close HLA match:

• Improves the chances for a successful transplant.
• Promotes engraftment. Engraftment is when the donated cells start to grow and make new blood cells. 
• Reduces the risk of a post-transplant complication called graft-versus-host disease (GVHD). GVHD occurs when the immune cells from the donated marrow or cord blood (the graft) attack your body (the host).
  
  

HLA matching requirements

There are many HLA markers. Research has found that a small number of them are most important to transplant outcomes. The NMDP sets minimum matching levels that must be met before a donor or cord blood unit from its Be The Match Registry can be used for a transplant. The 6 HLA markers looked at for these minimum requirements are two A, two B and two DRB1 markers (see Figure 1). 

For adult donors, the NMDP requires a match of at least 5 of these 6 HLA markers. For cord blood units — which require less strict matching criteria — the NMDP requires a match of at least 4 of these 6 markers. These minimum requirements are based on research studies of transplant outcomes.

Figure 1. HLA Matching of Patient and Donor

• Example A shows all the patient's markers match the donor's. The 6 of 6 match means that there is a match at A, B and DRB1. A 10 of 10 match means that there is a match at A, B, DRB1, C and DQ. 
• Example B shows that one of the patient's A markers does not match one of the donor's A markers. Therefore, this is a 5 of 6 match or a 9 of 10 match.

Most transplant centers have additional match requirements

After the NMDP's minimum matching requirements have been met, your transplant center will request additional tests to determine if the match is suitable for your transplant.

For example, your center might tell you they are searching for a "10 of 10" match for you. This means that they are looking at five pairs of markers. Besides the HLA pairs at A, B and DRB1, your center is also looking at other markers, such as HLA-C and HLA-DQ. That makes a total of ten markers. (See Transplant Center HLA Match Requirements to learn about matching requirements at specific transplant centers.)

Each marker can also be defined at a higher level of detail using DNA-based testing. The NMDP recommends that when possible, patients and unrelated donors should be matched at high resolution for HLA-A, B, C, and DRB1. Sometimes a patient and donor at first appear to have matching markers, but further testing shows that they do not match at a detailed level. Your transplant center will try to find a donor or cord blood unit that matches you at a detailed level.

To streamline the selection process, the NMDP uses state-of-the-art technology to help transplant centers predict which donors are most likely to match an individual patient when tested at high resolution. NMDP's HLA experts are also available to assist your transplant team if your search is especially challenging.

Other factors

The level of HLA matching is only one of the things that can affect your chances of having a successful transplant. The best available donor or cord blood unit may match you at all or at most of the HLA markers your transplant center looks at. Many patients who receive an unrelated donor transplant have a partially matched donor or cord blood unit.

For cord blood transplants, the number of blood-forming cells in the cord blood unit is important. The number of blood-forming cells in a transplant needs to be suitable for the size of the patient — larger patients need more blood-forming cells.

If your doctor finds more than one possible adult donor for you, your doctor will look at other factors. Doctors also may look at a donor's age, sex, blood type and size, the number of times a female donor has been pregnant and whether the donor tests positive for a common virus called cytomegalovirus (CMV).

How quickly you need a transplant and your disease status, your age and your general health can also affect your transplant center's strategy when choosing a donor or cord blood unit.

Challenging searches

Some patients face a greater challenge in finding a suitable donor or cord blood unit than others. At a detailed level, some HLA types are more common than others. In addition, some types are found more often in some racial and ethnic groups than others. Because HLA types are inherited, your best chance of finding a suitable donor may be with someone of a similar racial or ethnic background.

It is important to remember that you are not responsible for finding your own donor. Experts at your transplant center and the NMDP will conduct a worldwide search of the more than 13 million donors and more than 400,000 cord blood units on global listings.

And ongoing recruitment efforts add more than 48,000 new donors to the Be The Match Registry each month. Your formal search is updated daily, and doctors are notified as soon as a new donor is added who matches you. Our goal is to help your doctor find you the best match in the shortest amount of time.

In general, we encourage patients and their immediate families to focus their energies on caring for the patient. However, some families want to get involved in recruiting donors to the Be The Match Registry. If you want to raise awareness about the need for donors, we can help you — learn more.

Your potential matches on the Be The Match Registry

If you are not working with a transplant center, you may find NMDP's MatchView^® a helpful resource. You can use MatchView to view potential matches for your HLA type. MatchView can show you how many potential donors and cord blood units you may have on the registry.