Whether someone is waiting for a bone marrow match or hopes for a stem cell transplant, there are relatively few factors that matter in determining who would be a match. We’ll address those here.
The first requirement is that the donor be a match to the patient’s tissue type. The human leukocyte antigen or HLA tissue type is the most important. Every donor in the bone marrow match registry provides a DNA sample that is tested for six basic HLA markers. Anyone who is a good match based on those criteria for a patient will be sent for additional tests; whoever has the closest HLA markers is considered the best match.
The most likely match in terms of HLA is going to be a family member. The second most likely will be someone with the same ethnic background. This is why the bone marrow registry has been desperately seeking ethnic minority donors – because there are many willing white donors but their bone marrow just isn’t a match for many minorities who need such donations.
The fact that you are always a match with yourself has been one justification for storing cord blood from newborns. There have already been several bone marrow transplants using a child’s own cord blood. Cord blood from siblings conceived as “saviors” for a sibling with blood cancer is an ethically gray area, but it has saved a number of lives. Cord blood harvesting is being touted as an investment in your child’s health, too, because it is a safe source of cells for stem cell transplant for the child at any point in their lives.
If someone is in need of a bone marrow transplant procedure priority will be given to donors with the same blood type as the recipient. After receiving a bone marrow transplant from someone of another blood type, the donor’s body may change blood type over time to match that of the new bone marrow.
Donor health is always a factor to consider when determining who can be a donor. Bone marrow donation doesn’t pose the same health risk that donating an organ would, merely inconvenience and discomfort. However, the donor’s poor health could still preclude them donating bone marrow or stem cells. For example, someone with a deadly disease like HIV or Hepatitis C cannot be a bone marrow donor unless the recipient also has that disease.
Donor age has been found to be associated with long term survival and disease-free status for recipients of bone marrow, and it is probably relevant in other stem cell transplants as well. Your bone marrow and organs are as old as you are. You may be in excellent health for a 60 year old, but the recipient’s odds of survival are higher if they receive a transplant from an equally good match who is 26. A scientific study found that five year survival rates for recipients were 33% if the donor was under 30, 29% for donors between 31 and 45, and 25% if the donor was over 45. For patients receiving bone marrow from someone who wasn’t as good of an HLA match, survival rates were 28% when donors were under 30, 22% if donors were under 45, and less than 20% if donors were over 45. The impact of age on the survival rate of the recipient would mean that it is better to select a sibling donor over a parent and a child over a sibling.
Donor sex turned out to be a complicating factor. A female donor who had multiple pregnancies seemed to have trace amounts of her children’s stem cells in her body that carried over to the recipient. Recipients of their bone marrow suffered graft-versus-host disease at a rate of 55% versus 44% for everyone else. Solutions to this include preferring a male donor over a female one, the female who has never been pregnant, or selecting the youngest female donor on the assumption they’ve had fewer children.