The HLA and ABO Systems in the View of Stem Cell Transplant (HLA Typization: Choice of Donors)

The MHC is a large and stable region mapped to the short arm of the chromosome 6, encoding genes that have a lot of functions in immune and nonimmune response. MHC includes regions for the well-known and the most polymorphic until now discovered gene system, the human leukocyte antigen (HLA) genes. The extreme polymorphism of the HLA system derives from the existence of multiple alleles at several loci. It is estimated that more than 100 million different phenotypes can result from all combinations of alleles in the HLA system [1–5]. HLA genes are autosomal with codominant expression, inherited regularly as a haplotype. Since progeny receives one chromosome (haplotype) from each parent, four combinations of haplotypes are possible in newborn.