Medical Microbiology

FunctionsofMHCmolecules.MHCclassIandIImoleculesfunctionmainlyas

moleculescapableofpresentingpeptides(Figs.2. 7 –2. 9 ).Thesetwoclassesof

MHCmoleculespresenttwodifferenttypesofantigens:

— Intracellularantigens;thesearecleavedintopeptideswithintheprotea-

someandareusuallyassociatedwithMHCclassImoleculesviatheen-

dogenousantigenprocessingpathway(Fig.2. 8 ,leftside).

— Antigenstakenupfromexogenoussources;theseareprocessedinto

peptideswithinphagolysosomes,andinmostcasesarethenpresented

onMHCclassIImoleculesonthecellsurface(Fig.2. 8 ,rightside).Within

thephagolysosome,afragmentcalledtheinvariantchain(CLIP,classII-

inhibitingprotein)isreplacedbyanantigenfragment.ThisCLIPfragment

normallyblockstheantigen-bindingsiteoftheMHCclassIIdimer,thus

preventingitsoccupationbyotherintracellularpeptides.

ThepresentationgrooveofMHCclassImoleculesisclosedatbothends,and

onlyaccommodatespeptidesofroughly 8 – 10 (usually9)aminoacidsin

60 2 BasicPrinciplesofImmunology

ProteinStructureofMHCClassIMolecules

a

b

β 2 M N

α 1 α 2

N

N

C

C

α 3

Fig.2. 7 MHCclassItranslationantigen:alateralview,bverticalview.Thepre-

sentingpeptideisshowninviolet.Thethreedomainsoftheheavychainarea 1 ,a2,

anda 3 .b 2 microglobulin(b 2 M)functionsasalightchain,andisnotcovalently

boundtotheheavychain.

2

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Kayser, Medical Microbiology © 2005 Thieme

All rights reserved. Usage subject to terms and conditions of license.