1) Antibodies are antigen-binding proteins produced by B cells and plasma cells in response to antigens. The immune system responds to antigens by producing antibodies that target multiple epitopes on the antigen from several recruited B cell clones. 2) There are five major classes of antibodies (immunoglobulins): IgG, IgM, IgA, IgE, and IgD. Each class has unique structural and functional properties conferred by their constant region amino acid sequences. 3) IgG is the most abundant immunoglobulin and exists as four subclasses that differ in chain sequences and effector functions like complement activation and binding to Fc receptors. IgM is the first antibody produced and most efficient at activating complement.

1. Immunoglobulins
Submittedby
Nomananwar
15-arid-4488
Antibodiesare antigenbindingproteinspresentonBcell membrane andsecretedbyplasmacells.
The antibodiesproducedinresponse toaparticularantigenare heterogeneous.

2. Most antigensare complex andcontainmanydifferentantigenicdeterminants andthe immune system
usuallyrespondsbyproducingantibodiestoseveral epitopesonthe antigen.Thisresponse requiresthe
recruitmentof several clonesof Bcells.Theiroutputsare monoclonal antibodies,eachof which
specificallybindsasingle antigenicdeterminant .Together these monoclonal antibodiesmake upthe
polyclonal andheterogeneousserumantibodyresponsetoanimmunizingantigen.
Antibody Classes and Biological Activities
Each class isdistinguishedbyuniqueaminoacidsequencesinthe heavy-chainconstantregionthat
conferclass-specificstructural andfunctional properties.

4. ImmunoglobulinG(IgG)
IgG, the most abundantclassinserum,constitutesabout 80% of the total serumimmunoglobulin.The
IgG molecule consistsoftwoheavychainsandtwolightchain.
lightchains.There are fourhuman IgG subclassesdistinguishedbydifferencesin -chainsequence and
numberedaccordingtotheirdecreasingaverage serumconcentrations:IgG1,IgG2,IgG3,andIgG4 . The
aminoacid sequencesthatdistinguishthe fourIgGsubclassesare encodedbydifferentgerm-line CH
genes,whose DNA sequencesare 90%–95% homologous.The structural characteristicsthatdistinguish
these subclassesfromone anotherare the size of the hinge regionandthe numberandpositionof the
interchaindisulfide bondsbetweenthe heavychains.
IgG1 IgG3 andIgG4 readilycrossthe placentaandplayan importantrole inprotectingthe developing
fetus.
IgG3 isthe most effective complementactivator,followed byIgG1; IgG2 is lessefficient andIgG4 isnot
able to activate complementatall
IgG1 and IgG3 bindwithhighaffinitytoFcreceptorson phagocyticcellsandthusmediate
opsonization.IgG4has an intermediate affinityforFcreceptors andIgG2 has an extremelylow affinity.
ImmunoglobulinM (IgM)
IgMaccounts for 5%–10% of the total serumimmunoglobulin,withanaverage serumconcentrationof
1.5 mg/ml.MonomericIgM,witha molecularweightof 180,000, isexpressedasmembrane-bound

5. antibodyonB cells.IgMissecretedbyplasmacellsasa pentamer inwhichfive monomerunitsare held
togetherbydisulfide bondsthatlinktheircarboxyl-terminalheavychaindomains.
IgMis the firstimmunoglobulinclassproducedinaprimaryresponse toan antigen.
IgMis alsomore efficientthanIgG at activatingcomplement.ComplementactivationrequirestwoFc
regionsinclose proximity andthe pentamericstructure ofasingle molecule ofIgMfulfillsthis
requirement.Because of itslarge size,IgMdoesnotdiffuse well andtherefore isfoundinverylow
concentrationsinthe intercellulartissue fluids.The presence of the JchainallowsIgMto bind to
receptorsonsecretorycells whichtransportitacrossepithelialliningstoenterthe external secretions
that bathe mucosal surfaces.AlthoughIgA isthe majorisotype foundinthesesecretions IgMplaysan
importantaccessoryrole asa secretoryimmunoglobulin.
ImmunoglobulinA(IgA)
AlthoughIgA constitutesonly10%–15%of the total immunoglobulininserum, itisthe predominant
immunoglobulinclassinexternalsecretionssuchasbreastmilk,saliva,tears,andmucus of the
bronchial,genitourinary anddigestivetracts.

6. The dailyproductionof secretoryIgA isgreaterthan thatof any otherimmunoglobulinclass.IgA-
secretingplasmacellsare concentratedalongmucousmembranesurfaces.Alongthe jejunumof the
small intestine,forexample,there are more than2.5 1010 IgA-secretingplasmacells—anumberthat
surpassesthe total plasmacell populationof the bone marrow lymph andspleencombined!Everyday,a
humansecretesfrom5 g to 15 g of secretoryIgA intomucoussecretion.
ImmunoglobulinE (IgE)
The potentbiological activityof IgEallowedittobe identifiedinserumdespite itsextremelylow
average serumconcentration(0.3g/ml).IgEantibodiesmediatethe immediatehypersensitivityreactions
that are responsible forthe symptomsof hayfever,asthma,hives,andanaphylacticshock.The presence
of a serumcomponentresponsible forallergicreactions.
IgE bindsto Fc receptorsonthe membranesof bloodbasophilsandtissue mastcells

7. ImmunoglobulinD(IgD)
IgD has a serum concentration of 30 g/ml and constitutes about 0.2% of the total immunoglobulin in
serum. IgD together with IgM is the major membranebound immunoglobulin expressed by mature B
cells and its role in the physiology of B cells is under investigation.No biological effector function has
been identified for IgD

8. Summary