Although the number of known HLA alleles increases

from year to year, click here now reaching almost 2000 alleles at HLA-B (Table 2), only part of this polymorphism is detected in individual populations because of typing and statistical limitations (i.e. variable levels of typing resolution, and generally low sample sizes). However, most human populations exhibit a high level of HLA diversity. Table 3 summarizes data on the variation in the number of classical HLA alleles according to two independent studies. For most loci (except genes coding for the α chains of class II molecules, which are less polymorphic), between 10 and 30 alleles are observed per population, the largest number being observed at HLA-B (mean ∼ 30–32). With the exception of the DPB1 locus, and populations that underwent rapid genetic drift (see below), HLA

alleles generally exhibit low to medium frequencies, and many of them are very rare (and hence, rarely detected). Actually, 60–70% of known classical HLA alleles have only been reported up to three times,44,45 suggesting that new allele variants are being generated on a regular and ongoing basis. For most HLA loci, allele frequency distributions are usually even (except in some cases), and Ribonucleotide reductase populations see more achieve very high heterozygosity levels. This is reflected by the elevated mean heterozygosity values found

at each locus (Table 3), the highest value being observed for HLA-B (∼ 91%). Actually, with the exception of HLA-DPB1, heterozygosity levels are often higher than expected for populations undergoing neutral evolution (i.e. only submitted to stochastic factors linked to the history of human populations, like genetic drift and migration),46–50 which is consistent with the action of natural selection favouring heterozygosis. This hypothesis is also confirmed at the molecular level: at all classical HLA loci except DPB1 (and, to a lesser extent, DQB1), most alleles observed within populations are distantly related from a molecular point of view, with often more than 20 diverging nucleotides among their DNA sequences at exon 2 (and exon 3, for HLA class I).51 These HLA loci may therefore be experiencing asymmetric balancing selection where heterozygous genotypes having molecularly distant alleles would have a higher fitness than heterozygous genotypes exhibiting closely related alleles.51 By contrast, classical selective neutrality tests (e.g. Ewens–Watterson tests) performed at the DPB1 locus generally indicate a neutral model of evolution.