Adaptive immune system.docx

1. Adaptive immunesystem
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Thisarticle isabout humanimmunity.Forthe same conceptinbotany,see Plant-inducedsystemic
resistance.
A scanningelectronmicroscope imageof asingle humanlymphocyte
The adaptive immune system,alsoknownasthe acquiredimmune system, isasubsystemof the
immune systemthatiscomposedof specialized,systemiccellsandprocessesthateliminatepathogens
or preventtheirgrowth.The acquiredimmune systemisone of the twomainimmunitystrategiesfound
invertebrates(the otherbeingthe innateimmunesystem).
Google Ngramof "acquiredimmunity"vs."adaptive immunity".The peakfor"adaptive"inthe 1960s
reflectsitsintroductiontoimmunologybyRobertA.Goodanduse by colleagues;the explosive increase
inthe 1990s was correlatedwiththe use of the phrase "innate immunity".
Like the innate system,the adaptive immune systemincludesbothhumoral immunitycomponentsand
cell-mediatedimmunity componentsanddestroysinvadingpathogens.Unlike the innate immune
system,whichispre-programmedtoreactto commonbroad categoriesof pathogen,the adaptive
immune systemishighlyspecifictoeachparticularpathogenthe bodyhasencountered.
Adaptive immunitycreatesimmunological memoryafteraninitial response toaspecificpathogen,and
leadstoan enhancedresponse tofuture encounterswiththatpathogen.Antibodiesare a critical part of
the adaptive immune system.Adaptive immunitycanprovide long-lastingprotection,sometimesforthe
person'sentire lifetime.Forexample,someone whorecoversfrommeaslesisnow protectedagainst
measlesfortheirlifetime;inothercasesitdoesnotprovide lifetime protection,aswithchickenpox.This
processof adaptive immunityisthe basisof vaccination.
The cellsthat carry out the adaptive immune response are whitebloodcellsknownaslymphocytes.B
cellsandT cells,twodifferenttypesof lymphocytes,carryout the mainactivities:antibodyresponses,
and cell-mediatedimmune response.Inantibodyresponses,Bcellsare activatedtosecrete antibodies,
whichare proteinsalsoknownasimmunoglobulins.Antibodiestravel throughthe bloodstreamandbind
to the foreignantigencausingittoinactivate,whichdoesnotallow the antigentobindtothe host.[1]
Antigensare anysubstancesthatelicitthe adaptive immune response.Sometimesthe adaptivesystem

2. isunable to distinguishharmful fromharmlessforeignmolecules;the effectsof thismaybe hayfever,
asthma,or any otherallergy.
In adaptive immunity,pathogen-specificreceptorsare "acquired"duringthe lifetime of the organism
(whereasininnate immunitypathogen-specificreceptorsare alreadyencodedinthe genome).This
acquiredresponse iscalled"adaptive"because itpreparesthe body'simmune systemforfuture
challenges(thoughitcanactuallyalsobe maladaptive whenitresultsinallergiesorautoimmunity).
The systemishighlyadaptable because of twofactors.First,somatichypermutationisaprocessof
acceleratedrandomgeneticmutationsinthe antibody-codinggenes,whichallowsantibodieswithnovel
specificitytobe created.Second,V(D)Jrecombinationrandomlyselectsone variable(V),one diversity
(D),and one joining(J) regionforgeneticrecombinationanddiscardsthe rest,whichproducesahighly
unique combinationof antigen-receptorgene segmentsineachlymphocyte.Thismechanismallowsa
small numberof geneticsegmentstogenerate avastnumberof differentantigenreceptors,whichare
thenuniquelyexpressedoneachindividual lymphocyte.Since the generearrangementleadstoan
irreversible change inthe DNA of eachcell,all progeny(offspring) of thatcell inheritgenesthatencode
the same receptorspecificity,includingthe memoryBcellsandmemoryT cellsthatare the keysto long-
livedspecificimmunity.
Contents
1 Naming
2 Functions
3 Lymphocytes
4 Antigenpresentation
4.1 Exogenousantigens
4.2 Endogenousantigens
5 T lymphocytes
5.1 CD8+ T lymphocytesand cytotoxicity
5.2 HelperT-cells
5.2.1 Th1 andTh2: helperTcell responses
5.3 Gamma deltaT cells
6 B lymphocytesandantibodyproduction

3. 7 Alternativesystems
7.1 In jawlessvertebrates
7.2 In insects
8 Immunological memory
8.1 Passive memory
8.2 Active memory
8.2.1 Immunization
9 Immunological diversity
10 Acquiredimmunityduringpregnancy
11 Immune networktheory
11.1 Stimulationof adaptiveimmunity
12 Evolution
13 Typesof acquiredimmunity
14 See also
15 Notesandreferences
Naming
The term "adaptive"wasfirstusedbyRobertGoodin reference toantibodyresponsesinfrogsasa
synonymfor"acquiredimmune response"in1964. Good acknowledgedhe usedthe termsassynonyms
but explainedonlythathe preferredtouse the term"adaptive".He mighthave beenthinkingof the
thennot implausibletheoryof antibodyformationinwhichantibodieswere plasticandcouldadapt
themselvestothe molecularshape of antigens,and/ortothe conceptof "adaptive enzymes"as
describedbyMonodinbacteria,that is, enzymeswhose expressioncouldbe inducedbytheir
substrates.The phrase wasusedalmostexclusivelybyGoodandhisstudentsanda few other
immunologistsworkingwithmarginal organismsuntil the 1990s whenitbecame widelyusedintandem
withthe term"innate immunity"whichbecameapopularsubjectafterthe discoveryof the Toll
receptorsysteminDrosophila,apreviouslymarginal organismforthe studyof immunology.The term
"adaptive"asusedinimmunologyisproblematicasacquiredimmune responsescanbe bothadaptive
and maladaptive inthe physiological sense.Indeed,bothacquiredandinnate immune responsescanbe
bothadaptive andmaladaptive inthe evolutionarysense.Mosttextbookstoday,followingthe earlyuse
by Janeway,use "adaptive"almostexclusivelyandnotinginglossariesthatthe termissynonymouswith
"acquired".
The classicsense of "acquiredimmunity"came tomean,since Tonegawa'sdiscovery,"antigen-specific
immunitymediatedbysomaticgene rearrangementsthatcreate clone-definingantigenreceptors".In

4. the last decade,the term"adaptive"hasbeenincreasinglyappliedtoanotherclassof immune response
not so-farassociatedwithsomaticgene rearrangements.These includeexpansionof natural killer(NK)
cellswithso-farunexplainedspecificityforantigens,expansionof NKcellsexpressinggerm-line encoded
receptors,andactivationof otherinnate immune cellstoanactivatedstate that confersa short-term
"immune memory".Inthissense,"adaptiveimmunity"more closelyresemblesthe conceptof "activated
state"or "heterostasis",thusreturninginsense tothe physiological senseof "adaptation"to
environmental changes.
Functions
Overviewof the processesinvolvedinthe primaryimmune response
Acquiredimmunityis triggeredinvertebrateswhenapathogenevadesthe innateimmunesystemand
(1) generatesathresholdlevel of antigenand(2) generates"stranger"or"danger"signalsactivating
dendriticcells.[2]
The major functionsof the acquiredimmune systeminclude:
Recognitionof specific"non-self"antigensinthe presenceof "self",duringthe processof antigen
presentation.
Generationof responsesthatare tailoredtomaximallyeliminate specificpathogensorpathogen-
infectedcells.
Developmentof immunological memory,inwhichpathogensare "remembered"throughmemoryB
cellsandmemoryT cells.
In humans,ittakes4-7 daysfor the adaptive immune systemtomounta significantresponse.[3]
Lymphocytes
Main article:Lymphocyte
T and B lymphocytesare the cellsof the adaptive immunesystem.The humanbodyhasabout2 trillion
lymphocytes,whichare 20–40% of white bloodcells;theirtotal massisaboutthe same as the brainor
liver.The peripheral bloodstreamcontainsonly2% of all circulatinglymphocytes; the other98% move
withintissuesandthe lymphaticsystem, whichincludesthe lymphnodesandspleen.[1] Inhumans,
approximately1–2%of the lymphocyte pool recirculateseachhourtoincrease the opportunityforthe
cellstoencounterthe specificpathogenandantigenthattheyreactto.[4]

5. B cellsandT cellsare derivedfromthe same multipotenthematopoieticstemcells,andlookidentical to
one anotheruntil aftertheyare activated.B cellsplayalarge role inthe humoral immune response,
whereas Tcellsare intimatelyinvolvedincell-mediatedimmuneresponses.Inall vertebratesexcept
Agnatha,B cellsandT cellsare producedbystemcellsinthe bone marrow.[5] T cell progenitorsthen
migrate fromthe bone marrow to the thymus,where theydevelopfurther.
In an adultanimal,the peripheral lymphoidorganscontainamixture of Band T cellsinat leastthree
stagesof differentiation:
Naive Band naive T cells,whichhave leftthe bone marrow orthymusandenteredthe lymphatic
system,buthave yetto encountertheirmatchingantigen
Effectorcellsthathave beenactivatedbytheirmatchingantigen,andare activelyinvolvedineliminating
a pathogen
Memorycells,the survivorsof pastinfections
Antigenpresentation
Main article:Antigenpresentation
Acquiredimmunityreliesonthe capacityof immune cellstodistinguishbetweenthe body'sowncells
and unwantedinvaders.The host'scellsexpress"self"antigens.Theseantigensare differentfromthose
on the surface of bacteriaor onthe surface of virus-infectedhostcells("non-self"or"foreign"antigens).
The acquiredimmune responseistriggeredbyrecognizingforeignantigeninthe cellularcontextof an
activateddendriticcell.
Withthe exceptionof non-nucleatedcells(includingerythrocytes),all cellsare capable of presenting
antigenthroughthe functionof majorhistocompatibilitycomplex(MHC) molecules.[5] Some cellsare
speciallyequippedtopresentantigen,andtoprime naive Tcells.Dendriticcells,B-cells,and
macrophagesare equippedwithspecial "co-stimulatory"ligandsrecognizedbyco-stimulatoryreceptors
on T cells,andare termedprofessionalantigen-presentingcells(APCs).
Several Tcellssubgroupscanbe activatedbyprofessional APCs,andeachtype of T cell isspecially
equippedtodeal witheachunique toxinormicrobial pathogen.The type of T cell activated,andthe
type of response generated,depends,inpart,onthe contextinwhichthe APCfirstencounteredthe
antigen.[2]
Exogenousantigens

6. AntigenpresentationstimulatesTcellstobecome either"cytotoxic"CD8+cellsor"helper"CD4+ cells.
Dendriticcellsengulf exogenouspathogens,suchasbacteria,parasitesortoxinsinthe tissuesandthen
migrate,viachemotacticsignals,tothe T cell-enrichedlymphnodes.Duringmigration,dendriticcells
undergoa processof maturationinwhichtheylose mostof theirabilitytoengulf otherpathogens,and
developanabilitytocommunicate withT-cells.The dendriticcell usesenzymestochopthe pathogen
intosmallerpieces,calledantigens.Inthe lymphnode,the dendriticcell displaysthese non-self antigens
on itssurface by couplingthemtoa receptorcalledthe majorhistocompatibilitycomplex,orMHC (also
knowninhumansas humanleukocyte antigen(HLA)).ThisMHC-antigencomplex isrecognizedbyT-cells
passingthroughthe lymphnode.Exogenousantigensare usuallydisplayedonMHC classII molecules,
whichactivate CD4+T helpercells.[2]
Endogenousantigens
Endogenousantigensare producedbyintracellularbacteriaandvirusesreplicatingwithinahostcell.
The host cell usesenzymestodigestvirallyassociatedproteinsanddisplaysthese piecesonitssurface
to T-cellsbycouplingthemtoMHC. Endogenousantigensare typicallydisplayedonMHC classI
molecules,andactivate CD8+ cytotoxicT-cells.Withthe exceptionof non-nucleatedcells(including
erythrocytes),MHCclassI isexpressedbyall hostcells.[2]
T lymphocytes
Main article:T cell
CD8+ T lymphocytesandcytotoxicity
Main article:CytotoxicTcell
CytotoxicTcells(alsoknownasTC, killerTcell,orcytotoxicT-lymphocyte (CTL)) are asub-groupof T
cellsthatinduce the deathof cellsthatare infectedwithviruses(andotherpathogens),orare otherwise
damagedor dysfunctional.[2]
Naive cytotoxicTcellsare activatedwhentheirT-cell receptor(TCR) stronglyinteractswithapeptide-
boundMHC classI molecule.Thisaffinitydependsonthe type andorientationof the antigen/MHC
complex,andiswhatkeepsthe CTL and infected cellboundtogether.[2] Once activated,the CTL
undergoesaprocesscalledclonal selection,inwhichitgainsfunctionsanddividesrapidlytoproduce an
army of “armed” effectorcells.ActivatedCTLthentravelsthroughoutthe bodysearchingforcellsthat
bearthat unique MHC ClassI + peptide.[citationneeded]
Whenexposedtothese infectedordysfunctional somaticcells,effectorCTLrelease perforinand
granulysin:cytotoxinsthatformporesinthe targetcell'splasmamembrane,allowingionsandwaterto
flowintothe infectedcell,andcausingitto burstor lyse.CTL release granzyme,aserine protease
encapsulatedinagranule thatenterscellsviaporestoinduce apoptosis(celldeath).Tolimitextensive

7. tissue damage duringaninfection,CTLactivationistightlycontrolledandingeneral requiresavery
strongMHC/antigenactivationsignal,oradditional activationsignalsprovidedby"helper"T-cells(see
below).[2]
On resolutionof the infection,mosteffectorcellsdie andphagocytesclear themaway—butafewof
these cellsremainasmemorycells.[5] Onalaterencounterwiththe same antigen,these memorycells
quicklydifferentiateintoeffectorcells,dramaticallyshorteningthe time requiredtomountaneffective
response.[citationneeded]
HelperT-cells
Main article:T helpercell
The T lymphocyte activationpathway.Tcellscontribute toimmune defensesintwomajorways:some
directand regulate immune responses;othersdirectlyattackinfectedorcancerouscells.[6]
CD4+ lymphocytes,alsocalled"helper"Tcells,are immune responsemediators,andplayanimportant
role inestablishingandmaximizingthe capabilitiesof the acquiredimmune response.[2] These cells
have no cytotoxicorphagocyticactivity;andcannot kill infectedcellsorclearpathogens,but,inessence
"manage"the immune response,bydirectingothercellstoperformthese tasks.
HelperT cellsexpressTcell receptors(TCR) thatrecognize antigenboundtoClassIIMHC molecules.The
activationof a naive helperT-cellcausesittorelease cytokines,whichinfluencesthe activityof many
cell types,includingthe APC(Antigen-PresentingCell) thatactivatedit.HelperT-cellsrequireamuch
milderactivationstimulusthancytotoxicTcells.HelperTcellscan provide extrasignalsthat"help"
activate cytotoxiccells.[5]
Th1 andTh2: helperTcell responses
Classically,twotypesof effectorCD4+T helpercell responsescanbe inducedbya professionalAPC,
designatedTh1andTh2, eachdesignedtoeliminate differenttypesof pathogens.The factorsthat
dictate whetheraninfectiontriggersaTh1 or Th2 type response are notfullyunderstood,butthe
response generateddoesplayanimportantrole inthe clearance of differentpathogens.[2]
The Th1 response is characterizedbythe productionof Interferon-gamma,whichactivatesthe
bactericidal activitiesof macrophages,andinducesBcellstomake opsonizing(markingfor
phagocytosis) andcomplement-fixingantibodies,andleadstocell-mediatedimmunity.[2] In general,
Th1 responsesare more effectiveagainstintracellularpathogens(virusesandbacteriathatare inside
hostcells).

8. The Th2 response ischaracterizedbythe release of Interleukin5,whichinduceseosinophilsinthe
clearance of parasites.[7] Th2alsoproduce Interleukin4,whichfacilitatesBcell isotype switching.[2] In
general,Th2responsesare more effective againstextracellularbacteria,parasitesincludinghelminths
and toxins.[2] Like cytotoxicTcells,mostof the CD4+ helpercellsdie onresolutionof infection,witha
fewremainingasCD4+ memorycells.
Increasingly,thereisstrongevidence frommouse andhuman-basedscientificstudiesof abroader
diversityinCD4+effectorT helpercell subsets.[8][9] RegulatoryT(Treg) cells,have beenidentifiedas
importantnegative regulatorsof adaptive immunityastheylimitandsuppressthe immunesystemto
control aberrantimmune responsestoself-antigens;animportantmechanismincontrollingthe
developmentof autoimmunediseases.[5] FollicularhelperT(Tfh) cellsare anotherdistinctpopulationof
effectorCD4+ T cellsthatdevelopfromnaive Tcellspost-antigenactivation.Tfhcellsare specializedin
helpingBcell humoral immunityastheyare uniquelycapable of migratingtofollicularBcellsin
secondarylymphoidorgansandprovide thempositive paracrine signalstoenable the generationand
recall productionof high-qualityaffinity-maturedantibodies.SimilartoTregs,Tfhcellsalsoplaya role in
immunological tolerance asanabnormal expansionof Tfhcell numberscanleadtounrestricted
autoreactive antibodyproductioncausingsevere systemicautoimmune disorders.[10]
The relevance of CD4+ T helpercellsishighlightedduringanHIV infection.HIV isable tosubvertthe
immune systembyspecificallyattackingthe CD4+T cells,preciselythe cellsthatcoulddrive the
clearance of the virus,but alsothe cellsthatdrive immunityagainstall otherpathogensencountered
duringan organism'slifetime.[5]
Gamma deltaT cells
Main article:Gamma deltaT cell
Gamma deltaT cells(γδT cells) possessanalternative Tcell receptor(TCR) asopposedtoCD4+ and
CD8+ αβT cellsandshare characteristicsof helperTcells,cytotoxicTcellsandnatural killercells.Like
other'unconventional'Tcell subsetsbearinginvariantTCRs,suchas CD1d-restrictednatural killerTcells,
γδ T cellsexhibitcharacteristicsthatplace themat the borderbetweeninnateandacquiredimmunity.
On one hand,γδT cellsmaybe consideredacomponentof adaptive immunityinthattheyrearrange
TCR genesviaV(D)Jrecombination,whichalsoproducesjunctional diversity,anddevelopamemory
phenotype.Onthe otherhand,however,the varioussubsetsmayalsobe consideredpartof the innate
immune systemwherearestrictedTCRor NK receptorsmaybe usedas a patternrecognitionreceptor.
For example,accordingtothisparadigm, large numbersof Vγ9/Vδ2T cellsrespondwithinhoursto
commonmoleculesproducedbymicrobes,andhighlyrestrictedintraepithelial Vδ1Tcellsrespondto
stressedepithelial cells.

9. B lymphocytesandantibodyproduction
Main articles:B cell andHumoral immunity
The B lymphocyte activationpathway.Bcellsfunctiontoprotectthe hostbyproducingantibodiesthat
identifyandneutralize foreignobjectslike bacteriaandviruses.[6]
B Cellsare the major cellsinvolvedinthe creationof antibodiesthatcirculate inbloodplasmaand
lymph,knownashumoral immunity.Antibodies(alsoknownasimmunoglobulin,Ig),are large Y-shaped
proteinsusedbythe immune systemtoidentifyandneutralizeforeignobjects.Inmammals,there are
five typesof antibody:IgA,IgD,IgE,IgG, andIgM, differinginbiological properties;eachhasevolvedto
handle differentkindsof antigens.Uponactivation,Bcellsproduce antibodies,eachof whichrecognize
a unique antigen,andneutralizingspecificpathogens.[2]
Antigenandantibodybindingwouldcause fivedifferentprotectivemechanisms:
Agglutination:Reducesnumberof infectiousunitstobe dealt with
Activationof complement:Cause inflammationandcell lysis
Opsonization:Coatingantigenwithantibodyenhancesphagocytosis
Antibody-dependentcell-mediatedcytotoxicity:Antibodiesattachedtotargetcell cause destructionby
macrophages,eosinophils,andNKcells
Neutralization:Blocksadhesionof bacteriaandvirusestomucosa
Like the T cell,Bcellsexpressaunique Bcell receptor(BCR),inthiscase,amembrane-boundantibody
molecule.Allthe BCRof any one clone of B cellsrecognizesandbindstoonlyone particularantigen.A
critical difference betweenBcellsandT cellsishow each cell "sees"anantigen.Tcellsrecognize their
cognate antigenina processedform – as a peptide inthe contextof anMHC molecule,[2] whereasB
cellsrecognize antigensintheirnativeform.[2] Once aB cell encountersitscognate (orspecific) antigen
(andreceivesadditional signalsfromahelperTcell (predominatelyTh2type)),itfurtherdifferentiates
intoan effectorcell,knownasaplasmacell.[2]
Plasmacellsare short-livedcells(2–3days) that secrete antibodies.These antibodiesbindtoantigens,
makingthemeasiertargetsforphagocytes,andtriggerthe complementcascade.[2] About10% of
plasmacellssurvive tobecome long-livedantigen-specificmemoryBcells.[2] Alreadyprimedtoproduce
specificantibodies,these cellscanbe calledupontorespondquicklyif the same pathogenre-infectsthe
host,while the hostexperiencesfew,if any,symptoms.
Alternativesystems

10. In jawlessvertebrates
Main article:Adaptive immunityinjawlessvertebrates
Primitive jawlessvertebrates,suchasthe lampreyandhagfish,have anadaptive immune systemthat
shows3 differentcell lineages,eachsharingacommonoriginwithB cells,αβT cells,andinnate-likeγΔT
cells.[11][12] Insteadof the classical antibodiesandTcell receptors,these animalspossessalarge array
of moleculescalledvariablelymphocyte receptors(VLRsforshort) that,like the antigenreceptorsof
jawedvertebrates,are producedfrom onlyasmall number(one ortwo) of genes.These moleculesare
believedtobindpathogenicantigensinasimilarwaytoantibodies,andwiththe same degree of
specificity.[13]
In insects
For a longtime it wasthoughtthat insectsandotherinvertebratespossessonlyinnate immune system.
However,inrecentyearssome of the basichallmarksof adaptive immunityhave beendiscoveredin
insects.Those traitsare immune memoryandspecificity.Althoughthe hallmarksare presentthe
mechanismsare different fromthose invertebrates.
Immune memoryininsectswasdiscoveredthroughthe phenomenonof priming.Wheninsectsare
exposedtonon-lethal dose orheatkilledbacteriatheyare able todevelopamemoryof thatinfection
that allowsthemtowithstandotherwise lethal dose of the same bacteriatheywere exposedto
before.[14][15] Unlike invertebrates,insectsdonotpossesscellsspecificforadaptive immunity.Instead
those mechanismsare mediatedbyhemocytes.Hemocytesfunctionsimilarlytophagocytes andafter
primingtheyare able to more effectivelyrecognize andengulfthe pathogen.[16] Itwasalsoshownthat
it ispossible totransferthe memoryintooffspring.Forexample,inhoneybeesif the queenisinfected
withbacteriathenthe newlybornworkershave enhancedabilitiesinfightingwiththe same
bacteria.[17] Otherexperimental modelbasedonredflourbeetlealsoshowedpathogenspecificprimed
memorytransferintooffspringfrombothmothersandfathers.[18]
Most commonlyacceptedtheoryof the specificityisbasedonDscamgene.Dscam gene alsoknownas
Downsyndrome cell adhesive molecule isagene thatcontains3 variable Igdomains.Those domainscan
be alternativelysplicedreachinghighnumbersof variations.[19] Itwas shownthatafterexposure to
differentpathogensthereare differentsplice formsof dscamproduced.Afterthe animalswithdifferent
splice formsare exposedtothe same pathogenonlythe individualswiththe splice formspecificforthat
pathogensurvive.[19]
Othermechanismssupportingthe specificityof insectimmunityisRNA interference (RNAi).RNAi isa
formof antiviral immunitywithhighspecificity.[20] Ithasseveral differentpathwaysthatall endwith
the virusbeingunable toreplicate.One of the pathways issiRNA inwhichlongdoublestrandedRNA is
cut intopiecesthatserve as templatesforproteincomplexAgo2-RISCthatfindsanddegrades

11. complementaryRNA of the virus.MiRNA pathwayincytoplasmbindstoAgo1-RISCcomplex and
functionsasa template forviral RNA degradation.Lastone ispiRNA where small RNA bindstothe Piwi
proteinfamilyandcontrolstransposonesandothermobile elements.[21] Despite the researchthe exact
mechanismsresponsible forimmuneprimingandspecificityininsectsare not well described.
Immunological memory
Furtherinformation:Immunity(medical)
WhenB cellsandT cellsare activatedsome become memoryBcellsandsome memoryTcells.
Throughoutthe lifetime of ananimal these memorycellsformadatabase of effective B andT
lymphocytes.Uponinteractionwithapreviouslyencounteredantigen,the appropriatememorycellsare
selectedandactivated.Inthismanner,the secondandsubsequentexposurestoanantigenproduce a
strongerand fasterimmune response.Thisis"adaptive"inthe sense thatthe body'simmune system
preparesitself forfuture challenges,butis"maladaptive"of course if the receptorsare autoimmune.
Immunological memorycanbe inthe form of eitherpassive short-termmemoryoractive long-term
memory.
Passive memory
Passive memoryisusuallyshort-term,lastingbetweenafew daysandseveral months.Newborninfants
have had no priorexposure tomicrobesandare particularlyvulnerabletoinfection.Several layersof
passive protectionare providedbythe mother.Inutero,maternal IgGistransporteddirectlyacrossthe
placenta,sothat, at birth,humanbabieshave highlevelsof antibodies,withthe same range of antigen
specificitiesastheirmother.[2] Breastmilkcontainsantibodies(mainlyIgA) thatare transferredtothe
gut of the infant,protectingagainstbacterialinfections,untilthe newborncansynthesizeitsown
antibodies.[2]
Thisis passive immunitybecausethe fetusdoesnotactuallymake anymemorycellsorantibodies:It
onlyborrowsthem.Short-termpassiveimmunitycanalsobe transferredartificiallyfromone individual
to anotherviaantibody-richserum.
Active memory
In general,active immunityislong-termandcanbe acquiredbyinfectionfollowedbyBcell andT cell
activation,orartificiallyacquiredbyvaccines,inaprocesscalledimmunization.
Immunization
Historically,infectiousdiseasehasbeenthe leadingcause of deathinthe humanpopulation.Overthe
lastcentury,twoimportantfactorshave beendevelopedtocombattheirspread:sanitationand

12. immunization.[5] Immunization(commonlyreferredtoasvaccination) isthe deliberate inductionof an
immune response,andrepresentsthe singlemosteffective manipulationof the immunesystemthat
scientistshave developed.[5] Immunizationsare successfulbecause theyutilize the immune system's
natural specificityaswell asitsinducibility.
The principle behindimmunizationistointroduce anantigen,derivedfromadisease-causingorganism,
that stimulatesthe immune systemtodevelopprotective immunityagainstthatorganism, butthatdoes
not itself cause the pathogeniceffectsof thatorganism.Anantigen(shortforantibodygenerator),is
definedasanysubstance thatbindstoa specificantibodyandelicitsanadaptive immune response.[1]
Most viral vaccinesare basedon live attenuatedviruses,whereasmanybacterial vaccinesare basedon
acellularcomponentsof microorganisms,includingharmlesstoxincomponents.[1] Manyantigens
derivedfromacellularvaccines donotstronglyinduce anadaptive response,andmostbacterial
vaccinesrequire the additionof adjuvantsthatactivate the antigen-presentingcellsof the innate
immune systemtoenhance immunogenicity.[5]
Immunological diversity
An antibodyismade up of two heavychainsandtwo lightchains.The unique variableregionallowsan
antibodytorecognize itsmatchingantigen.[6]
Most large molecules,includingvirtuallyall proteinsandmanypolysaccharides,canserve asantigens.[2]
The parts of an antigenthatinteractwithan antibodymolecule oralymphocyte receptor,are called
epitopes,orantigenicdeterminants.Mostantigenscontainavarietyof epitopesandcanstimulate the
productionof antibodies,specificTcell responses,orboth.[2] A verysmall proportion(lessthan0.01%)
of the total lymphocytesare able tobindtoa particularantigen,whichsuggeststhatonlya few cells
respondtoeach antigen.[5]
For the acquiredresponse to"remember"andeliminate alarge numberof pathogensthe immune
systemmustbe able to distinguishbetweenmanydifferentantigens,[1] andthe receptorsthat
recognize antigensmustbe producedinahuge varietyof configurations,inessence one receptor(at
least) foreachdifferentpathogenthatmighteverbe encountered.Eveninthe absence of antigen
stimulation,ahumancanproduce more than 1 trilliondifferentantibodymolecules.[5] Millionsof genes
wouldbe requiredtostore the geneticinformationthatproducesthese receptors,but,the entire
humangenome containsfewerthan25,000 genes.[22]
Myriad receptorsare producedthrougha processknownas clonal selection.[1][2] Accordingtothe
clonal selectiontheory,atbirth,ananimal randomlygeneratesavastdiversityof lymphocytes(each

13. bearinga unique antigenreceptor) frominformationencodedinasmall familyof genes.Togenerate
each unique antigenreceptor,thesegeneshave undergone aprocesscalledV(D)Jrecombination,or
combinatorial diversification,inwhichone gene segmentrecombineswithothergene segmentstoform
a single unique gene.Thisassemblyprocessgeneratesthe enormousdiversityof receptorsand
antibodies,before the bodyeverencountersantigens,andenablesthe immunesystemtorespondtoan
almostunlimiteddiversityof antigens.[2] Throughoutananimal'slifetime,lymphocytesthatcanreact
againstthe antigensananimal actuallyencountersare selectedforaction—directedagainstanything
that expressesthatantigen.
Note that the innate andacquiredportionsof the immune systemworktogether,notinspite of each
other.The acquiredarm, B, andT cellscouldn'tfunctionwithoutthe innate systeminput.Tcellsare
uselesswithoutantigen-presentingcellstoactivate them, andB cellsare crippledwithoutTcell help.On
the otherhand,the innate systemwouldlikelybe overrunwithpathogenswithoutthe specializedaction
of the adaptive immune response.
Acquiredimmunityduringpregnancy
The cornerstone of the immune systemisthe recognitionof "self"versus"non-self". Therefore,the
mechanismsthatprotectthe humanfetus(whichisconsidered"non-self") fromattackbythe immune
system,are particularlyinteresting.Althoughnocomprehensive explanationhasemergedtoexplainthis
mysterious,andoftenrepeated,lackof rejection,twoclassical reasonsmayexplainhow the fetusis
tolerated.The firstisthatthe fetusoccupiesaportionof the bodyprotectedbya non-immunological
barrier,the uterus,whichthe immune systemdoesnotroutinelypatrol.[2] The secondis thatthe fetus
itself maypromote local immunosuppressioninthe mother,perhapsbyaprocessof active nutrient
depletion.[2] A more modernexplanationforthisinductionof tolerance isthatspecificglycoproteins
expressedinthe uterusduringpregnancy suppressthe uterine immuneresponse (seeeu-FEDS).
Duringpregnancyinviviparousmammals(all mammalsexceptMonotremes),endogenousretroviruses
(ERVs) are activatedandproducedinhighquantitiesduringthe implantationof the embryo.Theyare
currentlyknowntopossessimmunosuppressiveproperties,suggestingarole inprotectingthe embryo
fromits mother'simmune system.Also,viral fusionproteinscause the formationof the placental
syncytium[23] tolimitexchange of migratorycellsbetweenthe developingembryoandthe bodyof the
mother(somethinganepitheliumcan'tdo sufficiently,ascertainbloodcellsspecializetoinsert
themselvesbetweenadjacentepithelial cells).The immunodepressive actionwasthe initialnormal
behaviorof the virus,similartoHIV.The fusionproteinswereawayto spreadthe infectiontoother
cellsbysimplymergingthemwiththe infectedone (HIV doesthistoo).Itisbelievedthatthe ancestors
of modernviviparousmammalsevolvedafteraninfectionbythisvirus,enablingthe fetustosurvive the
immune systemof the mother.[24]
The human genome projectfoundseveral thousandERVsclassifiedinto24families.[25]

14. Immune networktheory
Main article:Immune networktheory
A theoretical frameworkexplainingthe workingsof the acquiredimmune systemisprovidedbyimmune
networktheory,basedoninteractionsbetweenidiotypes(unique molecularfeaturesof one clonotype,
i.e.the unique setof antigenicdeterminantsof the variable portionof anantibody) and'anti-idiotypes'
(antigenreceptorsthatreactwiththe idiotype asif itwere a foreignantigen).Thistheory,whichbuilds
on the existingclonal selectionhypothesisandsince 1974 has beendevelopedmainlybyNielsJerne and
GeoffreyW.Hoffmann,isseenas beingrelevanttothe understandingof the HIV pathogenesisandthe
searchfor an HIV vaccine.
Stimulationof adaptiveimmunity
One of the mostinterestingdevelopmentsinbiomedical science duringthe pastfew decadeshasbeen
elucidationof mechanisms mediatinginnate immunity.One setof innate immune mechanismsis
humoral,suchas complementactivation.Anothersetcomprisespatternrecognitionreceptorssuchas
toll-like receptors,whichinduce the productionof interferonsandothercytokinesincreasingresistance
of cellssuchas monocytestoinfections.[26] Cytokinesproducedduringinnate immune responsesare
amongthe activatorsof adaptive immune responses.[26] Antibodiesexertadditive orsynergisticeffects
withmechanismsof innate immunity. Unstable HbSclustersBand-3,amajorintegral redcell
protein;[27] antibodiesrecognize these clustersandaccelerate theirremoval byphagocyticcells.
ClusteredBand3 proteinswithattachedantibodiesactivate complement,andcomplementC3
fragmentsare opsoninsrecognizedbythe CR1 complementreceptoronphagocyticcells.[28]
A populationstudyhasshownthatthe protective effectof the sickle-cell traitagainstfalciparummalaria
involvesthe augmentationof acquiredaswell asinnate immune responsestothe malariaparasite,
illustratingthe expectedtransitionfrominnate toacquiredimmunity.[29]
Repeatedmalariainfectionsstrengthenacquiredimmunityandbroadenitseffectsagainstparasites
expressingdifferentsurface antigens.Byschool age mostchildrenhave developedefficaciousadaptive
immunityagainstmalaria.These observationsraise questionsaboutmechanismsthatfavorthe survival
of mostchildreninAfricawhile allowingsome todeveloppotentiallylethalinfections.
In malaria,asin otherinfections,[26] innate immune responsesleadinto,andstimulate,adaptive
immune responses.The geneticcontrol of innate andacquiredimmunityisnow alarge and flourishing
discipline.

15. Humoral and cell-mediatedimmune responseslimitmalariaparasite multiplication,andmanycytokines
contribute tothe pathogenesisof malariaaswell astothe resolutionof infections.[30]
Evolution
The acquiredimmune system,whichhasbeenbest-studiedinmammals,originatedinjawedfish
approximately500 millionyearsago.Mostof the molecules,cells,tissues,andassociatedmechanisms
of thissystemof defenseare foundincartilaginousfishes.[31] Lymphocyte receptors,IgandTCR, are
foundinall jawedvertebrates.The mostancientIgclass,IgM,is membrane-boundandthensecreted
uponstimulationof cartilaginousfishBcells.Anotherisotype,sharkIgW,isrelatedtomammalianIgD.
TCRs, bothα/βand γ/δ,are foundinall animalsfromgnathostomestomammals.The organizationof
gene segments thatundergogene rearrangementdiffersincartilaginousfishes,whichhave acluster
formas comparedto the transloconforminbonyfishto mammals.Like TCRand Ig,the MHC is found
onlyinjawedvertebrates.Genesinvolvedinantigenprocessingandpresentation,aswell asthe classI
and classII genes,are closelylinkedwithinthe MHCof almostall studiedspecies.
Lymphoidcellscanbe identifiedinsome pre-vertebrate deuterostomes(i.e.,seaurchins).[32] These
bindantigenwithpatternrecognitionreceptors(PRRs) of the innate immunesystem.Injawlessfishes,
twosubsetsof lymphocytesuse variable lymphocytereceptors(VLRs) forantigenbinding.[33] Diversity
isgeneratedbya cytosine deaminase-mediatedrearrangementof LRR-basedDNA segments.[34] There
isno evidence forthe recombination-activatinggenes(RAGs)thatrearrange Igand TCR gene segments
injawedvertebrates.
The evolutionof the AIS,basedonIg,TCR, and MHC molecules,isthoughttohave arisenfromtwo
majorevolutionaryevents:the transferof the RAG transposon(possiblyof viral origin) andtwowhole
genome duplications.[31] Thoughthe moleculesof the AISare well-conserved,theyare alsorapidly
evolving.Yet,acomparative approachfindsthatmanyfeaturesare quite uniformacrosstaxa.All the
majorfeaturesof the AISarose earlyandquickly.Jawlessfisheshave adifferentAISthatreliesongene
rearrangementtogenerate diverse immune receptorswithafunctional dichotomythatparallelsIgand
TCR molecules.[35] The innate immunesystem,whichhasanimportantrole inAISactivation,isthe
mostimportantdefense systemof invertebratesandplants.
Typesof acquiredimmunity
Immunitycanbe acquiredeitheractivelyorpassively.Immunityisacquiredactivelywhena personis
exposedtoforeignsubstancesandthe immune systemresponds.Passive immunityiswhenantibodies
are transferredfromone hosttoanother.Both activelyacquiredandpassivelyacquiredimmunitycan
be obtainedbynatural or artificial means.

16. NaturallyAcquiredActive Immunity –whenapersonisnaturallyexposedtoantigens,becomesill,then
recovers.
NaturallyAcquiredPassive Immunity –involvesanatural transferof antibodiesfromamotherto her
infant.The antibodiescrossthe woman'splacentatothe fetus.Antibodiescanalsobe transferred
throughbreastmilkwiththe secretionsof colostrum.
ArtificiallyAcquiredActiveImmunity –isdone byvaccination(introducingdeadorweakenedantigento
the host's cell).
ArtificiallyAcquiredPassive Immunity –Thisinvolvesthe introductionof antibodiesratherthanantigens
to the humanbody.These antibodiesare fromananimal or personwhoisalreadyimmune tothe
disease.
Naturallyacquired Artificiallyacquired
Active – Antigenentersthe bodynaturally Active – Antigensare introducedinvaccines.
Passive –Antibodiespassfrommothertofetusviaplacentaorinfantviathe mother'smilk. Passive
– Preformedantibodiesinimmuneserumare introducedbyinjection.
See also
WikimediaCommons hasmediarelatedtoAdaptiveimmunity.
Affinitymaturation
Allelicexclusion
Anergy
Immune response
Immune tolerance
Immunosuppression
Original antigenicsin
Somatichypermutation
Polyclonal response
Notesandreferences
Notes
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vte
Lymphocyticadaptive immunesystem andcomplement
vte
Organs of the lymphaticsystem
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Human systemsandorgans
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