WhyisImmunologicalMemoryNecessary?
Ahostwhichdoesnotsurviveaninitialinfectionobviouslydoesnotrequirefurther
immunologicalmemory.Ontheotherhand,survivaloftheinitialinfectionproves
thatthehost’simmunesystemcancontrolordefeattheinfection,onceagainap-
parentlynegatingtheneedforimmunologicalmemory.Evenassumingthatbetter
immunedefensesprovideaclearevolutionaryadvantage,especiallyduringpreg-
nancy,theideaofimmunologicalmemorymustbeunderstoodasprotectionwithin
adevelopmentalframework:
1 .DuetoMHCrestrictionofT-cellrecognition,itisnotpossibleforamothertopass
onT-cellimmunologicalexperiencetoherprogenyasthehistoincompatibility
reactionwouldinducemutualcellularrejection.Forthesamereason,achild’sT
cellsapparentlycannotmatureuntilrelativelylateinitsdevelopment(usually
aroundthetimeofbirth).Thisexplainswhynewbornsarealmostentirelylacking
inactiveimmunedefenses(Fig.2. 18 ).Newbornmicerequireaboutthreetofour
weeks(humansthreetoninemonths)beforetheT-cellimmuneresponseandthe
processofT-Bcellcollaborationwhichresultsinthegenerationofantibodyre-
sponsesbecomefullyfunctional.Duringthisperiodpassiveimmuneprotection
isessential.Thistypeofprotectionismediatedbythetransferofprotective,largely
IgG,antibodiesfrommothertochildthroughtheplacentaduringpregnancy,and
tosomeextentwithinthemother’smilk.Anexampleofthisisprovidedbycattle
wheretheacquisitionofcolostralmilkbythecalfisessentialtoitssurvival.Calves
canonlyaccessprotectiveIgGthroughthecolostralmilkdeliveredduringthefirst
24 hoursafterbirth(fetalcalfserumcontainsnoIg).Duringthefirst 18 hourspost
partum,thecalf’sintestineexpressesFcreceptorswhichallowtheuptakeof
undigestedantibodiesfromthemothersmilkintothebloodstream.Howcancom-
prehensive,transferable,antibody-mediatedprotectionbeensuredunderthese
conditions?Duringathree-weekmurineor 2 70-dayhumanpregnancy,mothers
donotnormallyundergoallofthemajortypesofinfection(indeedinfection
canbepotentiallylife-threateningforboththeembryo/fetusandthemother),
andsothearrayofantibodiesrequiredforcomprehensiveprotectioncannotbe
accumulatedduringthisperiodalone.Instead,anaccumulationoftheimmuno-
logicalprotectiveantibodylevelsrepresentingtheimmunologicallifeexperience
ofinfectionsinthemother’sserumisnecessary.Thefemalesexhormonesalso
encourageIgsynthesis,correlatingwithwomen’shigherrisklevel(aboutfivefold)
fordevelopingautoantibodydiseases(e.g.,lupus),andforautoimmunediseasesin
general.
2.Reproductionrequiresarelativelygoodlevelofhealthandagoodnutritional
statusofthemother.However,italsorequiresaneffectiveimmunedefensestatus
withinthepopulation(herd),includingmales,sinceallwouldotherwisebe
threatenedbyrepeatedandsevereinfections.Theincreasedfrequencyofspecific
precursorBandTcellsimprovesimmunedefensesagainstsuchinfections.How-
ever,thisrelativeprotectionisinclearcontrasttotheabsoluteprotectionan
immunoincompetentnewbornrequirestosurvive.ImmunologicalMemory 972All rights reserved. Usage subject to terms and conditions of license
Kayser, Medical Microbiology © 2005 Thieme
All rights reserved. Usage subject to terms and conditions of license.