Medical Microbiology

systemalsocontainsnon-specificdefensemechanisms,includingthecom-

plementsystem(see“Immuneresponseandeffectormechanisms,”p. 66 ff.).

Inchemicalterms,B-cellreceptorsareglobulins(Igorimmunoglobulins).

Theseimmunoglobulinscompriseanumberofclassesandsubclasses,as

wellasnumerousdifferentspecificities,butshareacommonstructure

(Fig.2.3a). &

ImmunoglobulinStructure

Allimmunoglobulinmonomershavethesamebasicconfiguration,inthat

theyconsistoftwoidenticallightchains(L)andtwoidenticalheavychains

(H).Thelightchainsappearastwoforms;lambda(k)orkappa(j).Thereare

fivemainheavychainvariants;l,d,c,a,ande.Thefivecorrespondingim-

munoglobulinclassesaredesignatedasIgM,IgD,IgG,IgA,orIgE,depending

onwhichtypeofheavychaintheyuse(Fig.2.3b).Aspecialcharacteristicof

theimmunoglobulinclassesIgAandIgMisthatthesecompriseabasic

monomericstructurethatcanbedoubledorquintupled(i.e.,thesecanexist

inadimericorpentamericform).Table2. 2 showsthecomposition,mole-

cularweightsandserumconcentrationsofthevariousimmunoglobulin

classes(p. 4 9).

50 2 BasicPrinciplesofImmunology

Fig.2. 3 aImmunoglobulinmonomers.Theupperhalfofthefigureshowsthe

intactmonomerconsistingoftwoLandtwoHchains.Thepositionsofthedis-

ulfidebonds,thevariableN-terminaldomains,andtheantigen-bindingsite(ABS)

areindicated.Thelowerhalfofthefigureshowsthemonomersoftheindividual

polypeptidechainsasseenfollowingexposuretoreducingconditions(whichbreak

thedisulfidebonds)anddenaturingconditions;notethattheABSislost.Papain

digestionproducestwomonovalentFabfragments,andoneFcfragment.Follow-

ingpepsindigestion(right),theFcportionisfragmented,buttheFabfragments

remainheldtogetherbydisulfidebonds.TheF(ab’) 2 armisbivalent(withtwo

identicalABS).Fvfragmentscompriseasingle-chainABSformedbyrecombinant

technology.TheseconsistofthevariabledomainsoftheHandLchains,joined

covalentlybyasyntheticlinkerpeptide.

bClassesofimmunoglobulins.IgM,IgD,IgG,IgA,andIgEaredifferentiated

bytheirrespectiveheavychains(l,d,c,a,e).IgA(achain)formsdimersheld

togetherbytheJ(joining)chain;thesecretory(S)piecefacilitatestransportof

secretoryIgAacrossepithelialcells,andimpairsitsenzymaticlysiswithinsecre-

tions.IgM(lchain)formspentamerswith 10 identicalABS;theIgMmonomers

areheldtogetherbyJchains.Thelightchains(kandj)arefoundinallclassesof

immunoglobulins. "

2

Kayser, Medical Microbiology © 2005 Thieme