Medical Microbiology

chainsofIgbearingthesamedesignations).Bothchainshavetwoextracel-

lulardomains,atransmembraneanchorelementandashortintracellularex-

tension.AsforIg,theterminaldomainsarevariableinnature(i.e.,VaandVb),

andtogethertheyformtheantigenbindingsite(seeFig.2. 9 ,p.65).T-cell

receptorsareassociatedwiththeirso-calledco-receptors—othermem-

brane-enclosedproteinsexpressedontheTcellsurface—whichinclude

themultiple-chainCD3complex,andCD4orCD8molecules(depending

onthespecificdifferentiationoftheTcell).CDstandsfor“clusterofdiffer-

entiation”or“clusterdeterminant”andrepresentsdifferentiationantigens

definedbyclustersofmonoclonalantibodies.(Table2. 13 ,p. 13 5f.,provides

asummaryofthemostimportantCDantigens.)

T-CellSpecificityandthe

MajorHistocompatibilityComplex(MHC)

T-cellreceptorsareunabletorecognizefreeantigens.InsteadtheT-cellre-

ceptorcanonlyrecognizeitsspecificepitopeoncetheantigenhasbeen

cleavedintoshorterpeptidefragmentsbythepresentingcell.Thesefrag-

mentsmustthenbeembeddedwithinaspecificmoleculargrooveandpre-

sentedtotheT-cellreceptor(aprocessknownasMHC-restrictedT-cellre-

cognitionorMHCrestriction).This“bindinggroove”islocatedontheMHC

molecule.TheMHCencodesforthepowerfulhistocompatibilityortrans-

plantationantigens(alsoknowninhumansasHLA,humanleukocyteantigen

molecules,Fig.2. 6 ).

Thedesignation“MHCmolecule”derivesfromtheinitialdiscoveryof

thefunctionofthecomplexasacellsurfacestructure,responsibleforthe

58 2 BasicPrinciplesofImmunology

TheMHCGeneComplex

HLA gene sequence

Chromosome 6

Allele variants

(approximate count)

Class

DPBDP

A

DQBDQADRBDRA C4BCYP2

1

C4AC2B1HSP70TNF B C A G

388 1914 691 6118 41

II III I

Fig.2. 6 Thehumanmajorhistocompatibilitygenecomplex(HLAgenes)islo-

catedonchromosome6.TherearethreedifferentclassesofMHCmolecules.

2

Kayser, Medical Microbiology © 2005 Thieme