The major histocompatibility complex: the value of extended haplotypes in the analysis of associated immune diseases and disorders.

Major histocompatibility complex antigens are critical to an animal's immune response. In most animals, the extreme polymorphism of MHC molecules complicates studies of the role of this complex in the immune response. In mice, however, MHC haplotype-homozygous inbred strains have been developed which are invaluable in the study of the immune system and the search for immune response genes. The human MHC bears many similarities to its murine equivalent with regard to antigen structure and polymorphism; furthermore, a number of combinations of specific MHC alleles between HLA-B and HLA-DR/DQ (extended haplotypes) are found in people more commonly than predicted by individual allele frequencies. Over 30 percent of Caucasian haplotypes are extended haplotypes, and over 55 percent of individuals have at least one extended haplotype. Examples of the same extended haplotype, even in unrelated individuals, should either all have or lack any gene within the MHC region. The value of considering extended haplotypes in searching for associations between the MHC and diseases, or immune response, is shown in three examples: congenital adrenal hyperplasia, hepatitis B immunization, and transfusion-associated graft-versus-host disease.