repeatedinaregularpattern(lineare.g.,flagella,ortwo-dimensionale.g.,
viruses)withintervalsof 5 – 10 nm.Theseparacrystalline-patternedantigens
arecapableofinducingB-cellresponseswithoutcontact-dependentTcell
help.ThisprobablyoccursbymeansofmaximumIgreceptorcross-linking.
SuchB-cellresponsesareusuallyoftheIgMtype,sinceswitchingtodifferent
isotypeclassesiseitherimpossibleorveryinefficientintheabsenceofTcell
help.TheIgMresponseisofarelativelybriefduration(exhibitingahalf-lifeof
about 24 h),butcannonethelessbehighlyefficient.Examplesofthiseffi-
ciencyincludeIgMresponsesinducedbymanyviralenvelopeantigenswhich
bearneutralizing(“protective”)determinantsaccessibletothecorresponding
antibodies,andresponsestobacterialsurfaceantigens(e.g.,flagellae,lipo-
polysaccharides)orparasites.
TCells.......................................................
T-CellActivation........................................
TherearetwoclassesofTcells;Thelpercells(CD4+)andcytotoxicTcells
(CD8+).Table2. 5 summarizestherelianceofT-cellresponsesonthedose,
localization,anddurationofpresenceofantigen.T-cellstimulationviathe
TCR,accessorymoleculesandadhesionmoleculesresultsintheactivation
ImmuneResponsesandEffectorMechanisms 71
Table2. 5 DependenceofT-CellResponseonAntigenLocalization,Amount,
andDurationofPresence
Antigen T-cellresponse
Localization Amount Durationof
presence
Thymus Small-large Always Negativeselectionby
deletion
Blood,spleen,
lymphnodes
(secondary
lymphoid
organs)
Small
Small
Large
Large
Short(1day)
Long(7days)
Short
Long(> 10 days)
Noinduction
Induction
Noinduction
Exhaustiveinduction/
deletion(anergy?)
Peripheralnon
lymphoidtissue
Largeorsmall Alwaysorshort Ignorance,indifference
2
Kayser, Medical Microbiology © 2005 Thieme