Figure 87

Allelic polymorphism. Allelic polymorphism is a hallmark of the human leukocyte antigen (HLA) system. The extreme polymorphism of the HLA system is seen in the large numbers of different alleles that exist for the multiple major histocompatibility complex (MHC) loci. At any given locus, one of several alternative forms or alleles of a gene can exist. Because so many alleles are possible for each HLA locus, the system is extremely polymorphic. The currently accepted World Health Organization serologically defined alleles are shown here. Established HLA antigens are designated by a number following the letter that denotes the HLA locus (eg, HLA-A1 and HLA-B8). For example, by serologic techniques, 28 distinct antigens are recognized at the HLA-A locus, and 59 defined antigens at the HLA-B locus. Sequencing studies of the HLA-DRB1 gene have identified over 100 distinct alleles, and preliminary analysis indicates that this level of polymorphism will be as high for other loci such as HLA-B. MHC polymorphism ensures effective antigen presentation of most pathogens; however, clinically, MHC polymorphism complicates attempts to find histocompatible donors for solid organ transplantation.

Antigens listed in parentheses are the broad antigens, antigens followed by broad antigens in parentheses are the antigen splits.

Father Mother ab cd

0 0

Post a comment