HLA-identical related donors (typically siblings) are generally considered to produce superior transplant outcome results compared to HLA-matched unrelated donors.76 77 It is against this subset of donors that all other alternate donor types are compared with regard to transplant outcome.
Since there is a 50% chance of two siblings inheriting the same HLA haplotype from a parent, there is a 25% chance that they will inherit the same chromosome from each parent. Overall, about 30-35% of patients find an HLA-matched sibling donor for transplantation. Serologic typing and low-resolution DNA typing for HLA-A, -B, -C, -DR may be sufficient to identify an HLA-identical sibling when enough family members have been studied to demonstrate that the same haplotypes were inherited (identity by descent). If not initially typed, HLA-DP typing may also be performed to exclude the possibility of a crossover in the interval between DR/DQ and DP. If two siblings have not been shown to be HLA identical by descent, there is the possibility that they have not inherited the same HLA haplotypes, but rather different haplotypes that are only similar at the tested loci. In such cases, additional study of untested loci and DNA testing at high resolution are warranted. If not initially studied, the inclusion of HLA-C and -DP testing is often useful to identify distinct haplotypes. Although HLA-identical relatives are generally siblings, more distant relatives are sometimes found to be HLA identical. This may be suggested by the architecture of the family and should be addressed when HLA-identical siblings are not identified.
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