Medical Microbiology

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