Figure 1. Simplified map of the genetic organization of the HLA complex. The principal class-I genes are represented by ovals and class-II genes by thick rectangles. The classical class-I and class-II HLA genes are indicated by green, whereas nonclassical ones by blue. The TAP and LMP genes, and tapasin-encoding gene within the class-II region are shown by red thin rectangles. Within the class-III region only the genes encoding complement proteins are shown by yellow short rectangles.

Figure 2. HLA alleles have extremely high polymorphisms at both (a) class I and (b) class II loci. The polymorphisms correspond to the antigen-binding site of MHC molecules, which they encode. Data from European Bioinformatics Institute, April 2004 (www.ebi.ac.uk/imgt/hla).

Figure 3. HIV patients with more heterozygous HLA loci progress to HIV more slowly than those with more homozygous loci. Modified from Carrington M, Nelson GW, Martin MP et al. (1999) HLA and HIV-1: heterozygote advantage and B*35-Cw*04 disadvantage. Science 283: 1748–1752.

Figure 4. Couples sharing HLA haplotypes have longer interbirth intervals than other couples. Modified from Ober et al. (1992).

Figure 5. Trans-species polymorphisms. (a) For most human genes, the closest relative of an allelic variant is another human allele. This is because the diversity of most alleles arose after the evolutionary divergence of humans and chimpanzees. (b) However, with MHC genes, the closest resemblance is to a chimpanzee rather than a human allele. This suggests that the diversity of human MHC alleles is ancient, originating before the divergence of humans and chimpanzees.