In this chapter, we have considered the theoretical and practical background of bone marrow transplantation. The immune response and its regulation by genes within the major histocompatibility complex, particularly of the I region of the mouse and of the HLA-D/DR region in man, is of central importance in both graft acceptance (rejection) and graft-versus-host disease. Methods which are available for typing alleles at the HLA-A, -C, -B, -DR and complotype (BF, C2, C4A, C4B) loci, have been considered in detail. The extent to which recombination affects specific alleles on haplotypes within families is discussed, as is the occurrence of linkage disequilibrium and extended haplotypes in populations of unrelated individuals. Because the HLA-DR and complotype region in man is thought to be critical for the success of bone marrow transplantation, methods for typing of HLA-D by both the HTC and PLT approaches have been examined. Although HLA-D/DR assignments are easily made in normal subjects, they are ambiguous in about 50 per cent of candidates for bone marrow transplantation, including, particularly, patients with aplastic anaemia, leukaemia, and severe combined immunodeficiency. In this setting, it is particularly important to obtain additional information by modification of HLA-D typing procedures and through complotype and GLO allele determinations in all family members. Finally, we can hope that there will be an increased possibility of using non-family donors through methods for removing cytotoxic T cells from donor marrow and through the identification, in the general population, of individuals who are genotypically similar or identical to the recipient. In this regard, the recognition that some 30 per cent of chromosome 6 in caucasians (50 per cent of individuals) bear extended haplotypes, which include a relatively fixed set of alleles particularly in the HLA-B, -DR, complotype and GLO regions, offers considerable promise.
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