Predictability of alloreactivity among unrelated individuals.

Extended haplotypes are specific HLA-B, BF, C2, C4A, C4B and DR allelic combinations that occur at high frequencies and show positive linkage disequilibrium among these highly polymorphic MHC markers. About 30% of all normal caucasian haplotypes are extended, and the matching of two extended haplotypes in unrelated individuals has been shown to match for the determinants of primary mixed lymphocyte reactivity (MLR-I). In this work we report that the matching of one extended haplotype and a serologically defined HLA-DR generic type on the second chromosome in unrelated individuals is associated with the absence of mixed lymphocyte reactivity in 15 to 30% of the cases studied. Our results suggest that, for those individuals who carry either one or two extended haplotypes, it is relatively easy to identify an unrelated MLR-I-matched subject. However, for individuals lacking at least one extended haplotype, it should be difficult to find an MLR-I-matched unrelated subject.