A bone marrow or hematopoietic stem cell transplant infuses healthy blood-forming stem cells into a patient’s body to replace cells damaged by disease or intensive cancer treatments. These procedures treat various conditions, including blood cancers like leukemia and lymphoma, and non-cancerous disorders like aplastic anemia and certain immune deficiencies. The goal is to replace the malfunctioning marrow with a new, healthy system that produces necessary blood cells. Finding a suitable donor is paramount, and the process begins by looking within the patient’s family.
Understanding HLA Tissue Typing
The most important factor in determining compatibility for a transplant is not blood type, but a genetic comparison called Human Leukocyte Antigen (HLA) tissue typing. HLA are proteins found on the surface of most cells, functioning as markers that help the immune system identify which cells belong. If the donor’s HLA markers do not closely match the recipient’s, the immune system will recognize the transplanted cells as foreign and attack them, leading to graft-versus-host disease (GvHD).
HLA markers are inherited from parents; each person receives one set of HLA genes from their mother and one set from their father. Compatibility is assessed across multiple markers, and a full match often requires a perfect score, such as 8 out of 8 or 10 out of 10 matched markers. Doctors use a blood test or a cheek swab to analyze the unique HLA combination of both the patient and potential donors. A closer match significantly increases the likelihood of a successful transplant and reduces the risk of severe post-transplant complications.
Matching Probabilities Among Family Members
Since HLA markers are inherited, a patient’s immediate family members are statistically the most likely compatible donor. Full siblings who share both biological parents have the highest chance of a perfect match (25% probability). Even if a sibling is not a perfect match, there is a 50% chance they will be a half-match, sharing one of the two inherited sets.
Parents and children are always a half-match, referred to as haploidentical. This is because a child inherits exactly half of their HLA genes from each parent, guaranteeing a 50% match. While a full match is traditionally preferred, medical advances have increasingly made haploidentical transplants a viable option.
The chances of finding a full match rapidly decrease with more distant relatives because less shared genetic material exists. Half-siblings, who share only one parent, cannot be a full HLA match, but they may still be a half-match. For extended family members like cousins, aunts, or uncles, the probability of finding a suitably matched donor is generally very low.
Medical Criteria for Donors
Even if a family member is a perfect genetic match, they must meet strict medical guidelines to be approved for donation. These requirements ensure the safety of both the donor and the recipient. Potential donors must be in good overall health.
Age is a significant factor; most bone marrow registries prefer donors between 18 and 44, as cells from younger donors are associated with more successful transplant outcomes. For family-directed donations, age limits can be more flexible, but they still exist. Donors must undergo a comprehensive screening process, including a physical examination, detailed medical history review, and blood tests to check for infectious diseases.
A history of certain medical conditions can disqualify a potential donor, even if they are a perfect HLA match. Exclusions typically include autoimmune disorders, such as severe rheumatoid arthritis, bleeding disorders, and some cancers. These health requirements are non-negotiable because the donor’s well-being is prioritized and the recipient must receive healthy stem cells.
Searching Beyond Immediate Family
When a fully matched donor cannot be found within the immediate family, the search expands to global registries. Organizations like the National Marrow Donor Program (NMDP) manage registries containing millions of potential unrelated donors. The process involves comparing the patient’s HLA type against the database to find the closest possible match.
Stem cells are collected using one of two methods: Peripheral Blood Stem Cell (PBSC) donation or traditional bone marrow donation. PBSC donation involves filtering stem cells from circulating blood after the donor receives medication to increase the stem cell count, and it is the more common method today. Traditional bone marrow donation is a surgical procedure that extracts liquid marrow from the pelvic bone under anesthesia.
For patients who do not find a full match, medical advancements have made the haploidentical transplant a more widely used alternative. Using a half-matched family member, such as a parent or child, is now viable due to improved conditioning techniques and post-transplant care. This approach significantly expands the donor pool, especially for those from ethnic backgrounds underrepresented in registries.