Population genetics of the HL-A system. A summary of data from the Fifth International Histocompatibility Testing Workshop.

The HL A system is an antigenic polymorphism detectable on most human tissues except erythrocytes. At present two loci, LA and 4 have been identified. They have about 14 and 17 alleles respectively and are separated by a recombination fraction of 0.8%. The high level of polymorphism, reflected in the low frequency of homozygosity observed in the HL A system, about 13% in Europeans compared with 58% for the mean of ABO, MN, Rh, Fy and K, allows the system to contribute as much information to population studies as most other polymorphisms combined. HL A antigens are usually detected on peripheral lymphocytes in a complement dependent cytotoxic assay using sera from multiparous women or immunized donors as antibody. In the 5th International Histocompatibility Testing Workshop 27 laboratories typed 54 populations for HL A and other polymorphisms. The studies showed marked differences in antigen frequencies, sometimes characteristic of a group of populations, e.g. HL A3 and 8 in Caucasoids, absent in Mongoloids, sometimes peculiar to a single population, e.g. HL A9 in Yemenite Jews, HL A10 in Australia aborigines. Frequencies of haplotypes, pairs of antigens, one from each locus inherited as a unit, one from each parent, were seen to be characteristic for groups. Clines in haplotype frequencies were observed, e.g. the HL A1, 8 and 3,7 haplotypes declined from north to south in Caucasoids with HL A11,5 going in the opposite direction. Three antigens, absent in Caucasoids, were identified; two, MO* and MWA found only in Negroids and one, Malay mainly in Mongoloids. American Indians showed greatly increased homozygosity for HL A compared with other polymorphisms and with other groups suggesting strong selective pressure on the HL A system, possibly connected with disease resistance mechanisms