Annual Review of Immunology Volume 8 1990 by Annual Reviews

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Theaminotermini and carboxy termini of fl~-microglobulin and the HLA-A2 heavy chain are indicated by N and C, respectively. In the case of the heavychain the carboxyterminusis that producedby papain digestion of the membrane boundmolecule. Theloop of the ~3 domainwhichis implicated in bindingCD8is shownin black and is delineated by the solid arrows. Thepeptide binding grooveand the hypothesizedarea of interaction with the T cell receptor are indicated. This figure is adaptedfrom Figure 2a of Bjorkmanet al (10) with permissionfrom Nature329: 509.

M ¯ m,~,m HLA-B8 nmm. nmm= mm ! HLA-Bw42 Figure 10 Recombinational mechanisms that contribute to diversification alleles. Identity in shading represents identity in sequence. 2 by a localized cluster of six nucleotide substitutions that produceaminoacid changesat positions 94, 95, and 97 (86). 2 within this region showidentity to other HLA-Blocus alleles, suggesting that one subtype wasproducedfrom the other by an allelic conversion. 3(45, 87). Examplesare, first, the sequencein codons79-83 whichdeterminesthe Bw4epitope and is shared by certain HLA-Aand B alleles and, second, the segmentencodingthe ~ helix of HLA-Bw46 whichis clearly derived from HLA-Cwll(Figure 10).

Expressionof two rather than one MHC allele will thus increase the total numberof peptides that potentially can be presented. Asimilar argumentcan explain whythere are multiple codominantlyexpressed and divergent classical class-I loci: The more distinct moleculesare expressed,the morepeptides can be presented. g. For example,the increased frequency of heterozygotes gained by having 40 rather than 35 alleles in a populationis so smallthat it is difficult to see howit wouldmakea difference. Additional componentsto the selection can be perceived from further consideration of the biology of MHC molecules and the immunesystem.

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