Medical Microbiology

host,itisparadoxicallytheimmuneresponsethatisresponsibleforpathol-
ogyandillnessduetoitsabilitytodestroyinfectedhosttissue.
HepatitisBviralinfectionsinhumans(Table2. 10 ),andLCMVinfections
(lymphocyticchoriomeningitis)inmice,areamongstthemostthoroughly
studiedexamplesofthispotentiallynegativeconsequenceofprotectiveim-
muneresponses.Asimilarsituationisalsoobservedforthecellularimmune
responseagainstfacultativeintracellulartuberculosisandleprosybacilli
whichthemselveshaverelativelylowlevelsofpathogenicity(Table2. 9 ).A
healthyimmunesystemwillnormallybringsuchinfectiousagentsunder
controlefficiently,andtheimmunologicalcellandtissuedamage(whichoc-
cursinparallelwiththeeliminationofthepathogen)willbeminimal,en-
suringthatthereislittlebywayofpathologicalorclinicalconsequence.How-
ever,shouldtheimmunesystemallowtheseagentstospreadfurther,the
resultwillbeachronicimmunopathologicalresponseandresultanttissue
destruction—asseenduringhepatitisBaschronicoracuteaggressivehepatitis
andinleprosyasthetuberculoidform.Shouldarapidlyspreadinginfection
resultinexhaustionoftheTcellresponse,orshouldaninsufficientlevelof
immunitybegenerated,theinfectedhostwillbecomeacarrier.Thiscarrier
state,whichonlyoccursduringinfectionscharacterizedbyanabsentorlow-
levelofcytopathology,isconvincinglydemonstratedinhepatitisBcarriers
andsufferersoflepromatousleprosy.

ImmuneDefensesagainstInfectionandTumorImmunity 105

Table2. 10 HepatitisBVirus(HBV)Infection.Inter-relationsbetweenEfficient
AntigenPresentationbyMHCMolecules,T-CellResponses,Course
ofInfection,andClinicalPicture.DecreasedImmunocompetenceor
EnhancedHBVProliferationShiftstheBalanceTowardsanUnfavor-
ableOutcome;VaccinationShiftstheBalanceTowardsaFavorable
Outcome

Presentation
ofHBVantigen
byMHC

T-cell

response

Kineticsofinfection Clinicalphenotype

+++ Early HBVproliferationis
halted

Acutehepatitiswithor
withouticterus,dueto
hepatocytedamagebeing
minimal
+/- Late HBVproliferationishalted
toolate.Livercellsare
lysedbyCD8+Tcells

Acutetochronicaggressive

hepatitis

• None HBVproliferationisnot
  halted,butthereisno
  immunopathology

HealthyHBVcarrier

(latelivercellcarcinoma)

2

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Kayser, Medical Microbiology © 2005 Thieme

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