One crucial factor in the success of a blood stem cell transplant is the degree of match between the tissue characteristics (HLA) of donor and patient. Tissue characteristics are protein structures on the surfaces of all cells and occur in different combinations in every individual, much like fingerprints. The immune system recognizes from these protein structures whether or not a cell belongs to its own body. If it does, the cell is left intact. If not, it is destroyed. That is why it is essential to find a donor whose HLA characteristics match the patient’s as closely as possible. This way, the new immune system that develops from the donor’s stem cells will accept the patient’s own cells.
Because tissue characteristics are inherited, the best chance of finding a match between donor and patient is within families. Parents are normally only haploidentical, or ‘half-matching’ donors, as a child inherits half of its tissue characteristics from the mother and half from the father. The highest probability of a match is between siblings, which is why they are the first family members to be tested as potential donors.
If there are no matching donors in the patient’s family, the search continues with an attempt to find a compatible unrelated donor. Around the world, over 36 million people have now chosen to register as potential blood stem cell donors, over 10 million of which are within DKMS’s worldwide database. If no suitable donor is found there, the search is extended internationally.