Medical Microbiology

bythedirecteliminationofAPC(i.e.,bychangingtheantigenkinetics),or

indirectlyviacytokineeffects(seeFig.2. 14 ,p.78).Thus,thenamesuppressor

Tcellsuggestsaregulatoryfunctionthatinrealityisunlikelytoexist.

Ingeneral,moreneutralnames,suchasCD4+TcellsorCD8+Tcells,arepre-

ferable.Whereasthecytotoxiceffectorcellsinthespleenandlymphnodes

possessaheterodimeric(a+bchain)CD8+Tmolecule,thefunctionofCD8+T

cellsfoundintheintestinalwallandexpressingthea-homodimericCD8

moleculeremainsunclear.

cdTcells.Asforthehomologousabheterodimer,thecdT-cellreceptoris

associatedwiththeCD3complexwithinthecellmembrane.Thegeneticse-

quenceforthecanddchainsresemblesthatoftheaandbchains,however,

thereareafewnotabledifferences.Thegenecomplexencodingthedchainis

locatedentirelywithintheVandJsegmentsoftheachaincomplex.Asa

result,anyrearrangementoftheachaingenesdeletesthedchaingenes.

TherearealsofarfewerVsegmentsforthecanddgenesthanforthea

andbchains.Itispossiblethattheincreasedbindingvariabilityofthed

chainsmakesupforthesmallnumberofVsegments,asaresultnearly

theentirevariabilitypotentialofthecdreceptorisconcentratedwithin

thebindingregion(Table2. 4 ,p.53).Theaminoacidscodedwithinthisregion

arepresumedtoformthecenterofthebindingsite.

TcellswithcdreceptorsrecognizecertainclassI-likegeneproductsinas-

sociationwithphospholipidsandphosphoglycolipids.Inperipherallymphoid

tissues,onlyasmallnumberofTcellsexpressthecdandCD3co-receptor,

however,manyoftheTcellsfoundwithinthemucosaandsubmucosaex-

presscdreceptors.

cdTcellscanbenegativeforCD4+andCD8+,orexpresstwoachains(but

nobchain)oftheCD8+molecule.AlthoughitisassumedthatcdTcellsmay

beresponsibleforearly,low-specificity,immunedefenseattheskinandmu-

cosa,theirspecificitiesandeffectorfunctionsarestilllargelyunknown.

ImmuneResponsesandEffectorMechanisms

&Theeffectorfunctionsoftheimmunesystemcompriseantibodiesand

complement-dependentmechanismswithinbodyfluidsandthemucosa,

aswellastissue-boundeffectormechanismsexecutedbyTcellsandmono-

cytes/macrophages.Bcellsarecharacterizedbyantigenspecificity.Following

antigenstimulation,specificBcellsproliferateanddifferentiateintoplasma

cellsthatsecreteantibodiesintothesurroundings.ThetypeofB-cellre-

sponseinducedisdeterminedbytheamountandtypeofboundantigen

recognized.InductionofanIgMresponseinresponsetoantigenswhich

arelipopolysaccharides—orwhichexhibitanhighlyorganized,crystal-like

66 2 BasicPrinciplesofImmunology

2

Kayser, Medical Microbiology © 2005 Thieme