Medical Microbiology

stitutingpartofamacromolecule.Thisiswhytheepitopecomponentofan

antigenisterminologicallydistinguishedfromitsmacromolecularcarrier;

togethertheyformanimmunogen.Blymphocytesreacttotheantigensti-

mulusbyproducingantibodies.TheTlymphocytes(Tcells)responsiblefor

cellularimmunityarealsoactivated.Thesecellscanonlyrecognizeprotein

antigensthathavebeenprocessedbyhostcellsandpresentedontheirsur-

face.TheT-cellreceptorsrecognizeantigenfragmentswithalengthof 8 – 12

sequentialaminoacidswhichareeithersynthesizedbythecellitselforpro-

ducedsubsequenttophagocytosisandpresentedbythecellulartransplan-

tationantigenmoleculesonthecellsurface.TheTcellscanthencomplete

theirmaintask—recognitionofinfectedhostcells—sothatinfectionishalted.

Ourunderstandingoftheimmunedefensesystembeganwithstudiesof

infectiousdiseases,includingtheantibodyresponsestodiphtheria,dermal

reactionstotuberculin,andserodiagnosisofsyphilis.Characteriztionof

pathologicalantigensprovedtobeenormouslydifficult,andinsteaderythro-

cyteantigens,artificiallysynthesizedchemicalcompounds,andothermore

readilyavailableproteinswereusedinexperimentalmodelsformorethan 60

years.Majorbreakthroughsinbacteriology,virology,parasitology,biochem-

istry,molecularbiology,andexperimentalembryologyinthepast 30 – 40

yearshavenowmadeanewphaseofintensiveandproductiveresearchpos-

siblewithinthefieldofimmunedefensesagainstinfection.Theaimofthis

chapteronimmunology,inacompactguidetomedicalmicrobiology,isto

presenttheimmunesystemessentiallyasasystemofdefenseagainstin-

fectionsandtoidentifyitsstrengthsandweaknessestofurtherourunder-

standingofpathogenesisandpreventionofdisease.

TheImmunologicalApparatus

&Theimmunesystemiscomprisedofvariouscontinuouslycirculatingcells
(TandBlymphocytes,andantigen-presentingcellspresentinvarioustis-
sues).TandBcellsdevelopfromacommonstemcelltype,thenmature
inthethymus(Tcells)orthebonemarrow(Bcells),whicharecalledprimary
(orcentral)lymphoidorgans.Anantigen-specificdifferentiationstepthen
takesplaceswithinthespecializedandhighlyorganizedsecondary(orper-
ipheral)lymphoidorgans(lymphnodes,spleen,mucosa-associatedlym-
phoidtissues[MALT]).Theantigen-specificactivationofBand/orTcellsin-
volvestheirstaggeredinteractionwithothercellsinacontact-dependent
mannerandbysolublefactors.
Bcellsbearantibodiesontheirsurfaces(cell-boundB-cellreceptors).
Theysecreteantibodiesintotheblood(solubleantibodies)orontomucosal
surfacesoncetheyhavefullymaturedintoplasmacells.Antibodiesrecognize

TheImmunologicalApparatus 45

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Kayser, Medical Microbiology © 2005 Thieme