Medical Microbiology

taininglargenumbersofepitopessimilartotheAandBepitopes.Duringthe

firstmonthsoflife,peoplewithbloodgroupO(homozygousfortheOallele)

producebothanti-Aandanti-Bantibodies,peoplewithbloodgroupA(gen-

otypeAOorAA)produceonlyanti-Bantibodies,peoplewithbloodgroupB

(genotypeBOorBB)produceonlyanti-Aantibodies,andpeoplewithblood

groupABproduceneitheranti-Aoranti-Bantibodies.

Theseso-called“natural”antibodies(meaningtheseantibodiesarepro-

ducedwithoutarecognizableimmunizationprocess)areoftheIgMclass;

thereisusuallynoswitchtoIgG,probablyresultingfromalackofnecessary

helperT-cellepitopes.Thepresenceofthebloodgroupantibodiesmakes

bloodtransfusionsbetweennon-matchedindividualsextremelyrisky,neces-

sitatingthatthebloodgroupofboththedonorandrecipientisdetermined

beforethebloodtransfusiontakesplace.Nevertheless,theantibodiesinthe

donorbloodarenotsoimportantbecausetheyarediluted.TheOgenotypeis

thereforeauniversaldonor.NotethatIgMantibodiestobloodgroupspresent

nodangertothefetussincetheycannotpassthroughtheplacentalbarrier.

Rhesusfactor.Thissystemisalsobasedongeneticallydeterminedantigens

presentonredbloodcells,althoughasageneralrulethereisnoproductionof

“natural”antibodiesagainstthese.IgMandIgGantibodiesarenotinduced

unlessanimmunization(resultingfrombloodtransfusionorpregnancy)

takesplace.Duringthebirthprocess,smallamountsofthechild’sbloodoften

enterthemother’sbloodstream.Shouldthechild’sbloodcellshavepaternal

antigens,whicharelackinginthemother’sblood,hisorherbloodwilleffec-

tively’immunize’themother.ShouldIgGantibodiesdeveloptheywillrepre-

sentapotentialriskduringsubsequentpregnanciesshouldthefetusonce

againpresentthesameantigen.Theresultingclinicalpictureisknownas

morbushemolyticusneonatorumorerythroblastosisfetalis(“immunehydrops

fetalis”).

Onceimmunizationhasoccurred,thusendangeringfuturepregnancies,ge-

neticallyatriskchildrencanstillbesavedbymeansofcesareansectionand

exchangebloodtransfusions.Shouldtheriskofrhesusimmunizationbere-

cognizedattheendofthefirstpregnancy,immunizationofthemothercanbe

preventedbymeansofapassiveinfusionofantibodiesagainstthechild’santi-

gen,immediatelyfollowingthebirth.Thisspecificimmunosuppressivepro-

cedureisanempiricalapplicationofimmunologicalknowledge,althoughthe

precisemechanisminvolvedisnotyetbeencompletelyunderstood.

Otherbloodgroupsystems.Thereareotheradditionalbloodgroupsystems

againstwhichantibodiesmaybeproduced,andwhichcanpresentariskdur-

ingtransfusions.Thus,thecrossmatchtestrepresentsanimportantmeasure

intheavoidanceoftransfusionproblems.Immediatelypriortoaplanned

transfusion,serumfromtheprospectiverecipientismixedwitherythrocytes

fromtheprospectivedonor,andserumfromtheprospectivedonorismixed

112 2 BasicPrinciplesofImmunology

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Kayser, Medical Microbiology © 2005 Thieme

All rights reserved. Usage subject to terms and conditions of license.