Bone marrow is the soft, spongy tissue found inside certain bones, such as the hip bones, ribs, and sternum. It produces blood cells, including red blood cells, white blood cells, and platelets. Approximately 500 billion blood cells are produced each day.
When diseases or treatments damage the bone marrow, its ability to generate healthy blood cells becomes impaired. Bone marrow transplants replace diseased bone marrow with healthy stem cells, offering a potential cure for various life-threatening conditions. Finding a compatible donor is crucial for a successful transplant.
Understanding How Bone Marrow Is Matched
Matching a bone marrow donor to a patient relies on Human Leukocyte Antigens (HLAs). HLAs are proteins on the surface of most cells, central to the immune system. The immune system uses these markers to distinguish between the body’s own cells and foreign invaders.
If donor and patient HLA types do not closely match, the patient’s immune system may recognize donor cells as foreign. This can lead to severe complications like transplant rejection (patient’s immune system attacks transplanted cells) or graft-versus-host disease (GVHD, donor’s immune cells attack patient’s tissues). A close HLA match minimizes these risks, improving transplant success.
The HLA system is diverse, with thousands of variations, making an exact match challenging. These variations are inherited, with half of a person’s HLA genes from each parent.
Scientists categorize HLAs into Class I (HLA-A, -B, -C) and Class II (HLA-DRB1, -DQB1, -DPB1), which are most important for matching. Doctors assess how well a donor and patient match across multiple HLA markers, often looking for 8 to 10 out of 12 key markers, or a perfect 10 out of 10. More identical HLA markers shared between a donor and recipient lower the risk of complications like GVHD. Medical research continues to refine understanding of how specific mismatches might be tolerated to expand the pool of potential donors.
How to Get Tested to Determine Your Match Status
To determine your bone marrow match status, join a bone marrow donor registry. In the United States, a key registry is the National Marrow Donor Program (NMDP), also known as Be The Match. These registries maintain databases of potential donors and their HLA types for medical professionals to search.
The initial registration step often involves a cheek swab. A cotton swab collects cells from the inside of the cheek, which are then analyzed in a laboratory to determine your HLA type. Sometimes, a small blood sample is collected instead. This initial test provides the HLA typing information to add your profile to the registry.
Once registered, your HLA information becomes available for patients worldwide. If a preliminary match is identified, the registry will contact you for further, more detailed testing. This secondary testing involves additional blood samples or cheek swabs to confirm the HLA match at a higher resolution and conduct other health screenings. Joining a registry is the most direct way to find out if you could be a bone marrow match.
Understanding Match Probability and Donor Compatibility
Finding a perfect bone marrow match is challenging due to the vast diversity of HLA types. Genetic inheritance plays a significant role in compatibility, making siblings the most likely candidates for a successful match. A sibling sharing the same biological parents has about a 25% chance of being a full HLA match. Parents are rarely full matches with their children (typically less than 1%), though they are always a half-match.
If a fully matched family donor is unavailable (about 70% of patients), the search extends to unrelated donors through national and international registries. The probability of finding an unrelated match varies significantly (29% to 79%) depending on the patient’s ethnic background. This disparity exists because HLA types are inherited and vary considerably among different ethnic groups.
Patients from diverse ethnic backgrounds often face greater difficulty finding a compatible donor. For instance, white patients have about a 79% chance of finding a match, while African American patients may have as low as a 25-29% chance, and Hispanic or Asian patients have a 47-48% chance. This highlights the importance of increasing ethnic diversity within bone marrow donor registries to improve the chances of finding matches for all patients.