Medical Microbiology

(dermal)hypersensitivity(DTH,seebelowandp. 11 4f.)canbeapplieddiag-

nosticallytodetermineinfectionimmunity(forexampleagainsttuberculosis

andleprosy),sincetheexistenceofcontinuedinfectioncontinuouslyacti-

vatesthoseTcellsrequiredforbothpathogencontrolandDTHreactions.

DelayedDermalHypersensitivityReaction

Theclassicexampleofadelayedtypehypersensitivity(DTH)reactionisthe

tuberculinreaction(Mantouxtestinhumans).Itwasoneofthefirstspecific

cell-mediatedimmuneresponsestobeidentified—asearlyasthe 19 40singuinea

pigs.TheresponseisspecificforMHCclassIIantigensandisCD4+Tcell-dependent.

Insomecases,especiallyduringactiveviralinfections,aDTHreactionistransiently

observedandismediatedbyCD8+Tcells.ThesimplestwaytoelicitaDTHreaction

istointroduceadiagnosticprotein,obtainedfromthepathogen,intotheskin.The

testreactionwillonlydevelopshouldcontinuouslyactivatedTcellsbepresentwith-

inthehost,sinceonlythesecellsarecapableofmigratingtodermallocationswithin

24 – 48 hours.IfnoactivatedTcellsarepresent,re-activationwithinthelocallymph

nodesmustfirsttakeplace,andhencemigrationintothedermiswillrequiremore

time.Bythistimethesmallamountofintroduceddiagnosticpeptide,orprotein,

willhavebeendigestedorwillhavedecayedandthuswillnolongerbepresentat

theinjectionsiteinthequantityrequiredforinductionofalocalreaction.

Apositivedelayedhypersensitivityreactionis,therefore,anindicatorofthepre-

senceofactivatedTcells.Theabsenceofareactionindicateseitherthatthe

hosthadneverbeenincontactwiththeantigen,orthatthehostnolongerpos-

sessesactivatedTcells.Inthecaseoftuberculosis,anegativeskintestcanindicate

that;nomoreantigenorgranulomatissueispresent,orthatthesystemicimmune

responseismassiveandthepathogenisspreadthroughoutthebody.Inthelatter

case,theamountofdiagnosticproteinusedisnormallyinsufficientfortheattrac-

tionofresponsiveTcellstothesiteofinjection,andasaconsequencenomeasur-

ablereactionbecomesevident(sothattheMantouxtestmaybenegativeinLand-

ouzysepsisormiliarytuberculosis).DTHreactionsprovideadiagnostictestfortu-

berculosis(Mantouxtest),leprosy(lepromintest),andBoeck’ssarcoid(Kveim

test).However,thesedermalreactionsmaydisappearinthosepatientsthatare

immunosuppressedorinfectedwithmeaslesorAIDS.

ImmuneDefensesagainstInfectionandTumorImmunity.........

&Protectionagainstinfectionscanbemediatedbyeither;non-specificde-
fensemechanisms(interferons,NKcells),orspecificimmunityintheform
ofantibodiesandTcellswhichreleasecytokinesandmediatecontact-
andperforin-dependentcelllysis.Controlofcytopathicvirusesrequiresso-
lublefactors(antibodies,cytokines),whilstcontrolofnoncytopathicviruses

ImmuneDefensesagainstInfectionandTumorImmunity 99

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Kayser, Medical Microbiology © 2005 Thieme

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