Molecular histocompatibility typing in unrelated donor bone marrow transplantation

Increasing numbers of bone-marrow transplants are being performed with marrow from unrelated donors. There is clear evidence that genetic disparity is a major cause of graft-versus-host disease. However, until very recently, typing methods did not have sufficient resolution to identify the majority of histocompatibility alleles. DNA-based typing methods have the potential to define alleles unequivocally, and this will help in the selection of better matched marrow donors. Also, a number of studies have indicated that patient and donor pairs that had previously been identified as matched can frequently be shown to have been mismatched at one or more locus. Consequently, it may become more difficult in future to find completely matched donors. There is also evidence that some genetic mismatching may not be deleterious to the patient and it may now be possible to analyse the histocompatibility data with greater confidence and identify those mismatches that are tolerated. This will allow a larger number of transplants to be performed using unrelated donors.