Figure 93.2 Prototypic HLA-DR haplotypes. The number of HLA-DRB genes and expressed DR molecules varies with different haplotypes. In this figure five common or prototypic HLA haplotypes, the normally associated DR types, and number of expressed DR molecules are shown. ^ indicates a nonexpressed DRB pseudogene

Figure 93.3 Domain structure of HLA molecules. HLA class I and class II molecules are heterodimeric glycopro-teins composed of four external domains. The two most external domains of each molecule form a peptide binding structure that is recognized by T-cell receptors
Figure 93.4 Exon-intron organization of HLA class I and class II genes. Class I and class II genes are segmented into multiple exons separated by noncoding introns. Each external protein domain of the mature molecules is encoded by different exons

genes (Figure 93.4) corresponds in large part to the domain structure of the encoded molecules. Exons 2, 3, 4 of the class I alpha genes primarily encode the alpha 1, 2, 3 domains of the class I protein. In class II molecules, the alpha and beta gene products each contribute one domain to the peptide binding structure of the het-erodimer. HLA class I molecules are found on nearly all nucleated cells. Class II molecules have a more restricted distribution, being found on B lymphocytes, activated T lymphocytes, macrophages, and dendritic cells.

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