clonal selection theory.PPTX

PG AND RESEARCH DEPARTMENT OF LIBRARY ,SRI PARAMAKALYANI COLLEGE,ALWARKURICHI.

1. UNIT: I
CLONAL SELECTION THEORY

2. Synopsis
▪Introduction
▪Types of Immune Response
▪B lymphocytes (b cells)
▪Memory cells
▪Burnet’s clonal selection theory
▪According to clonal selection hypothesis

3. Clonal section
theroy

4. INTRODUCION
▪Clonalselectiontheoryisascientifictheoryin
immunologythatexplainsthefunctionsofcellsofthe
immunesystem(lymphocytes)inresponsetospecific
antigensinvadingthebody.
▪EachBcellhasaspecificantibodyasacellsurface
receptor.
▪Thearrangementandgenerationofantibodygenes
occurspriortoanyexposuretoantigen.

5. ▪Whenasolubleantigenispresent,itbindstothe
antibodyonthesurfaceofBcellsthathavethe
correctspecificity.
▪TheseBcellclonesdevelopintoantibodyproducing
plasmacellsormemory.
▪OnlyBcells,whichareantigen-specific,arecapable
ofsecretingantibodies.

7. Types of Immune Response
PrimaryImmuneResponse
ThisisaresponsetoaninvadertheFirsttimetheinvader
infectsthebody.
Nomeasurableimmuneresponseforfirstfewdays.
Next10-15daysantibodyproductiongrowssteadily

8. Secondary Response
Amorerapidresponsetoaninvaderthe2
nd
timeitinvadesthe
body.
Antibodyproductionincreasesdramaticallyandinamuch
shortertimeperiod

9. B lymphocytes (b cells)
▪Lymphocytesrespondspecificallytoantigenson
foreigncells,cellsinfectedbypathogensandtoxins
releasedbypathogens.
▪ANANTIGENISACOMPLEXMOLECULE—
RECOGNISABLE ASSELFORFOREIGN/NON-
SELF
▪ANTIGEN TRIGGERS PRODUCTION OF
ANTIBODIES
▪ANTIBODYISAY-SHAPEDMOLECULEWITH
ASPECIFICRECEPTOR(BINDINGSITE)

10. Memory cells
▪SomeTandBlymphocytesproducedinresponseto
antigensbyclonalselectionsurvivelongtermasmemory
cells.
▪Asecondaryexposuretothesameantigenrapidlygivesrise
toanewcloneoflymphocytesproducingarapidand
greaterimmunologicalresponse.

11. ▪.

12. How Do B Cells Produce Antibodies?
▪Bcellsdevelopfromstemcellsinthebonemarrowof
adults(liveroffetuses).
▪AftermaturationBcellsmigratetolymphoidorgans
(lymphnodeorspleen).

13. Clonal Selection:
▪WhenaBcellencountersanantigenitrecognizes,it
isstimulatedanddividesintomanyclonescalled
plasmacells,whichactivelysecreteantibodies.
▪EachBcellproducesantibodiesthatwillrecognize
onlyoneantigenicdeterminant.

15. BURNET’S CLONAL SELECTION THEORY
▪1959,F.M.Burnetproposedthe
clonalselectiontheoryofantibody
production.
▪Theclonalselectionhypothesis
statesthatanindividualBcell
expressesreceptorsspecifictothe
distinctantigen,determinedbefore
theantibodyeverencountersthe
antigen.
Sir Macfarlane Burnet

16. ▪The clonal selection hypothesis has become
a widely accepted model for how the immune
system responds to infection and how certain
types of B and T lymphocytes are selected
for destruction of specific antigens

17. According to clonal selection
hypothesis:
▪Thelymphoidstemcellsdifferentiaterandomlytoproduce
differentmaturelymphocyteeachcarryinganAgbinding
receptor.
▪Eachlymphocytebearsauniquetypeofreceptorwitha
uniquespecificity(derivedbyV(D)Jrecombination),
determinedbeforetheantibodyeverencounterstheantigen

19. ▪Theoccupationofareceptorbyanantigen(epitope)isrequired
forcellactivation(clonalselection),whichisfollowedby
proliferation(clonalexpansion)ofthiscelltoformclones.
▪TheactivatedcellsofaclonedifferentiateintoAbproducing
cells,effectorcells&memorycells.
▪Thedifferentiatedeffectorcellsderivedfromanactivated
lymphocyteclonewillbearreceptorsofidenticalspecificityas
theparentalcell.

20. ▪ThespecificityofAbproducedbyalymphocyteis
identicaltothatofitsAgreceptor.
▪Thoselymphocytesbearingreceptorsforself
moleculeswillbedeletedatanearlystage.

21. Steps of Clonal Selection:
▪Intheprimaryimmuneresponse,clonalselectionproduces
effectorcellsandmemorycellsthatmayconferlife-long
immunity.
▪Inthesecondaryimmuneresponse,memorycellsare
activatedbyasecondexposuretothesameantigen,which
initiatesarapidandanamnesticresponse.

22. ▪Randomsomatichypermutationsduringclonal
expansioncausetheproductionofBcellswith
increasedantibody-bindingaffinityfortheirantigens.
▪Theclonalselectionhypothesismayexplainwhy
secondaryimmuneresponsesaresoeffectiveat
preventingreinfectionbythesamepathogen.

24. ▪Itgenerallytakes4-5daysforanaiveBlymphocytethathas
beenactivatedtocompleteclonalexpansionanddifferentiate
intoeffectorB-lymphocytes.AlthoughBurnetproposedthis
theoryforBcellsandantibodyproduction,butitisequally
applicabletoTcellsalso.
▪Clone:Acloneisdefinedasagroupofidenticalcellsderived
fromasinglecell.

25. Clonalselection:Whenanantigenencounterstheimmune
system,itsepitopeseventuallywillreactonlywithB-
lymphocyteswithB-cellreceptorsontheirsurfacethatmore
orlessfitandthisactivatesthoseB-lymphocytes.This
processisknownasclonalselection.

26. Reference
▪Hodgkin,PhilipD;Heath,WilliamR;Baxter,AlanG
(2007)."Theclonalselectiontheory:50yearssincethe
revolution".NatureImmunology.8(10):1019–
1026PMID24141853.
▪Burnet,F.M.(1976)."AModificationofJerne'sTheoryof
AntibodyProductionusingtheConceptofClonal
Selection".CA:ACancerJournalforClinicians.2
▪Murphy,Kenneth(2012).Janeway'sImmunobiology8th
Edition. New York, NY: Garland
Science.ISBN9780815342434

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