医学免疫学：08主要主要组织相容性复合体及其编码分子.ppt

* * * Example: If MHC X was the only type of MHC molecule Population threatened with extinction Survival of individual threatened Pathogen that evades MHC X MHC XX Example: If each individual could make two MHC molecules, MHC X and Y Impact on the individual depends upon genotype Pathogen that evades MHC X MHC XX MHC XY Population survives MHC YY but has sequences that bind to MHC Y Example: If each individual could make two MHC molecules, MHC X and Y……and the pathogen mutates Population threatened with extinction Survival of individual threatened Pathogen that evades MHC X but has sequences that bind to MHC Y MHC XX MHC XY MHC YY The number of types of MHC molecule can not be increased ad infinitum ….until it mutates to evade MHC Y Populations need to express variants of each type of MHC molecule Populations of microorganisms reproduce faster than humans Mutations that change MHC-binding antigens or MHC molecules can only be introduced to populations after reproduction The ability of microorganisms to mutate in order to evade MHC molecules will always outpace counter evasion measures that involve mutations in the MHC The number of types of MHC molecules are limited To counteract the superior flexibility of pathogens: Human populations possess many variants of each type of MHC molecule Variant MHC may not protect every individual from every pathogen.However, the existence of a large number of variants means that the population is prevented from extinction YRYR XY XX XXR XYR YXR YYR YY XRXR XRYR From 2 MHC types and 2 variants……. 10 different genotypes Variants – alleles - of each type of MHC gene encode proteins that increase the resistance of the population from rapidly mutating or newly encountered pathogens without increasing the number of types of MHC molecule Variant MHC molecules protect the population Pathogen that evades MHC X and Y MHC XY MHC XX MHC YY MHC XXR MHC YYR …but binds to the variant MHC XR and MHC YR 6－4　蛋白质抗原的处理与递呈Processing an Presentation