Bone marrow transplantation requires a close match between the HLA tissue type of the patient and the donor. HLA stands for "human leukocyte antigen," a marker that the immune system uses to recognize which cells belong in the body and which do not. HLA type is not the same as blood type, and is more complex.
HLA type is inherited, with one half from one parent and one half from the other. Each child has a 25 percent chance of inheriting the same HLA type as their siblings. Due to this low probability, there are many children who do not have a brother or sister who is HLA-matched and eligible to donate marrow for a transplant. For children with sickle cell disease, finding a match can be even more difficult because many have siblings who also have sickle cell disease, making them unable to donate.
For children without a sibling match who have complications from sickle cell disease, an unrelated donor search can be conducted through the National Marrow Donor Program (also known as the Be The Match Registry) and other worldwide registries. This donor search can often find an unrelated volunteer donor with the same HLA typing. If a matched unrelated donor is located, these children can also be eligible to receive a transplant.