ABSTRACT
The HLA genetic region encodes histocompatibility molecules that play a central role in
the immune response. A key feature of the HLA system is its extensive polymorphism.
Availability of DNA-based typing methods for precise discrimination of HLA allelic vari-
ants has contributed to a more complete understanding of the immunogenetic barriers that
lead to graft-vs.-host disease (GVHD). Data demonstrate that the overall results of unre-
lated hematopoietic cell transplantation (HCT) can be optimized through comprehensive
and precise donor matching for HLA class I and II alleles. Furthermore, the permissibility
of an HLA mismatch is shaped by a complex interaction of qualitative and quantitative
differences in the HLA sequences between the donor and recipient. With the ability to
type and match donors with precision, clinical experience now demonstrates that the over-
all outcome of unrelated HCT can approach that of sibling transplantation, especially for
good risk patients. Furthermore, it is possible to achieve excellent clinical results with a
less than perfectly matched donor, thereby increasing the accessibility to donors for
patients in need of a transplant.
