Medical Microbiology

immunoglobulinmonomerhastwoidenticalantigen-bindingsites(ABS),
andtheseformtheendsofthetwoshortarmsofthe’Y’.Anareawithin
theantibodyconsistingof 12 – 15 aminoacidscontactsthepeptideregion
containedwithintheantigenandconsistingofapproximately 5 – 800 A ̊^2
(Table2. 3 ).Thetrunkofthe’Y’iscalledtheFcfragment(named,“fraction
crystallizable”sinceitcrystallizesreadily)andismadeupoftheconstant
domainsoftheheavychains(CH2andCH3,andsometimesCH4).

DiversitywithintheVariableDomains

oftheImmunoglobulins

Thespecificityofanantibodyisdeterminedbytheaminoacidsequenceof
thevariabledomainsoftheHandLchains,andthissequenceisuniquefor
eachcorrespondingcellclone.Howhasnaturegoneaboutthetaskofprodu-
cingtheneededdiversityofspecificaminoacidsequenceswithinabiochemi-
callyeconomicalframework?ThegeneticvarietycontainedwithintheB-cell
populationisensuredbyaprocessofcontinuousdiversificationofthegeneti-
callyidenticalB-cellprecursors.Thethreegenesegments(variable,diversity,
joining)whichencodethevariabledomain(theVDJregionfortheHchain,
andtheVJregionfortheLchain)arecapableofundergoingaprocesscalled
recombination.Eachofthesegeneticsegmentsarefoundasanumberofvar-
iants(Fig.2. 4 ,Table2. 4 ).B-cellmaturationinvolvesaprocessofgeneticre-

TheB-CellSystem 53

Table2. 4 OrganizationoftheGeneticRegionsfortheHumanImmunoglobu-
linsandT-CellReceptors(TCR)

Immunoglobulins TCRab TCRcd

H L a b c d

Vsegments 95 150 50 – 100 75 – 100 9 6

Dsegments 23 – – 2 – 3

Jsegments 9 12 60 – 80 13 5 3

Nucleotide
additions

VD,DJ VJ VJ VD,DJ VJ VD

Numberofpotential
combinationsforV
(H+L)

15000 8000 54

Theoreticalupperlimit
ofallcombinations

>1 012 >1 012 >1 012

2

Kayser, Medical Microbiology © 2005 Thieme