The choice of a syngeneic, autologous, or allogeneic HSCT is relatively predetermined for most patients, as few patients have an identical twin and the availability of an allogeneic donor is limited. In allogeneic HSCT, stem cells are obtained from someone (i.e., the donor) other than the patient (i.e., the recipient). Donor and recipient are generally identical or "matched" for HLA, which are derived from the MHC located on chromosome 6.15 A single set of MHC alleles, described as a haplotype, is inherited from each parent, resulting in HLA pairs. The most important HLA include HLA-A, HLA-B, HLA-C, DR, and DQ loci. Among siblings, the genes that encode for HLA-B and C are located so close to each other in the MHC that one is rarely inherited without the other. As a result, an HLA match among siblings is referred to a "6 of 6," as they are matched for HLA-A, B, and DR; however, in actuality they are matched for all of the HLA antigens.3 The other antigens, such as HLA-C, become more important when alternative sources of hematopoietic stem cells are used, such as those from unrelated donors and cord blood, which are described in more detail next and in Chapter 98.16

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