The dominant factor determining the likely incidence of host-versus-graft and GvHD is the compatibility between donor and recipient at the major histocompatibility complex (MHC) loci, a group of highly polymorphic genes clustered on chromosome 6 in the human and encoding, among
others, the sequence of the cell-surface molecules (HLA) presenting antigen to T cells. The MHC profile is now identified at an allelic level for the most important class I (HLA-A, -B, -C) and class II loci (HLA-DR and -DQ) (tissue typing). Ideally a donor should match at both alleles of each locus. By virtue of the inheritance of the MHC genes as a group (haplotype) (recombination events are unusual), sibling donors have a one in four chance of matching at all 10 loci. They are also more likely to match at a number of minor histocompatibility antigenic sites, making them a preferred choice over an unrelated donor with an equivalent MHC type.
