Variation In Hla Genetic Organization

Complexity of the HLA region is manifest at several levels. As noted above, each HLA chromosomal segment or haplotype has multiple class I and class II loci. However, there is variation in the number of HLA-DRB genes present on commonly occurring haplotypes. There are five different prototypic haplotypes that are very commonly observed9-11 (Figure 93.2). Thus, haplotypes expressing DR1, DR8, or DR10 typically have only the DRB1* gene that can be expressed, while most other haplotypes have DRB1* and a second gene expressed (DRB3*, DRB4*, or DRB5*, encoding DR52, DR53, and DR51 antigens, respectively). It should be noted that exceptions to these prototypic haplotypes are known.12-15 These exceptional haplotypes may be difficult to match in unrelated-donor searches. Further, some reasonably frequent haplotypes do not express a second DR antigen as a result of an inactivating mutation, e.g., HLA-B57, DRB1*0701, DRB4*01030102N, DQB1*03030216; DRB1*1502, DRB5*0108N, DQB1*050117; HLA-B52, DRB1*1502, DRB5*0110N, DQB1*0503.18

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