Section 4Section 4

• An enormous family of related, but nAn enormous family of related, but nonidentical glycoproteinsonidentical glycoproteins

Expression of IgExpression of Ig

• membrane Ig, mIg : expressen on the smembrane Ig, mIg : expressen on the surfaces of resting B lymphocytes, as reurfaces of resting B lymphocytes, as receptorceptor

• secreted Ig, sIg : as antibody(Ab)secreted Ig, sIg : as antibody(Ab)

Major functions of IgMajor functions of Ig

• Binding AgBinding Ag• Secondary biologic activitiesSecondary biologic activities

– OpsoninsOpsonins– Activate the complement cascadeActivate the complement cascade– Cross the placental barrierCross the placental barrier

Chapter 1Chapter 1 organization of organization of immunoglobulinsimmunoglobulins

Molecular structure of IgMolecular structure of Ig

• An enormous family of related, but nonideAn enormous family of related, but nonidentical glycoproteinsntical glycoproteins

• Ab are bifuctional molecules: Ab are bifuctional molecules:

bind specifically to Agbind specifically to Ag

secondary phenomenasecondary phenomena

The four-chain basic unitThe four-chain basic unit

Symmetrical structureSymmetrical structureTwo Heavy (H) chains, about twice aTwo Heavy (H) chains, about twice a

s large as the L chain s large as the L chain Two Light (L) chains, the smallerTwo Light (L) chains, the smallerCovalent interchain disulfide bridgeCovalent interchain disulfide bridge

ss

(Ag-biding site)

L chain

H chain(,,,,)

V region

C region

HH22LL22

hinge regiohinge regionn

2 regions2 regions

κ/λ (2:1)

Monomer of Ig

domainsdomains

Both H and L chains contain folded gloBoth H and L chains contain folded globular domainsbular domains

Each domains has a single intrachain Each domains has a single intrachain disulfide bonddisulfide bond

L chains always contain two domains L chains always contain two domains H chains contain either 4 or 5 domainsH chains contain either 4 or 5 domains

Immunoglobulin variable Immunoglobulin variable regions and constant regionsregions and constant regions

variable region (V region)variable region (V region)

The sequences of the N-The sequences of the N-terminal terminal

domain in a heavy- or light-chain domain in a heavy- or light-chain

polypeptide are variablepolypeptide are variable– VVHH

– VVL L

constant region (C region)constant region (C region)

• The sequences of the other The sequences of the other domains remain relatively domains remain relatively constantconstant– CCHH

– CCL L

• L-chain polypeptides contain L-chain polypeptides contain only a Conly a CL L domaindomain

• H-chain polypeptides contain 3 H-chain polypeptides contain 3 or more Cor more CH H domainsdomains

（二）可变区和恒定区

The VThe VHH is juxtaposed with the V is juxtaposed with the VL L to fto form an Ag-binding siteorm an Ag-binding site

Each Each monomer of Ig contains two monomer of Ig contains two AAg-binding sitesg-binding sites

Hinge regionHinge region

Located between the CH1 and CH2 oLocated between the CH1 and CH2 of the H chains in most Igf the H chains in most Ig

Permits flexibility between the two Permits flexibility between the two Fab arms of AbFab arms of Ab

Allows the two arms to open and cloAllows the two arms to open and close to accommodate binding to two se to accommodate binding to two epitopesepitopes

Flexibility of immunoglobulins

Hypervariable regionsHypervariable regions

The V regions contain two regions:The V regions contain two regions:

Framework regions----relatively invarFramework regions----relatively invariantiant

Hypervariable regions----extremely vHypervariable regions----extremely variant ariant Complementarity-determining rComplementarity-determining regions (CDRs)egions (CDRs)

Complementarity-determining regionComplementarity-determining regions (CDRs)s (CDRs)

Each VH and VL contains 3 CDRsEach VH and VL contains 3 CDRs

Structure of the Structure of the Variable DomainVariable Domain

Relation to AgRelation to Ag--bindingbinding

Antigen-binding Can Take Many Forms – Best Method

J chainJ chain IgM and IgA generally exist as polymIgM and IgA generally exist as polym

ersers

Joint Joint monomers to form polymers

n(Hn(H22LL22)) n(monomer)n(monomer)

J chainJ chain (H(H22LL22)n)n JJ

polymerpolymer

J chain and secretary J chain and secretary componentcomponent

Secretory componentSecretory component

• Associated only with IgA and is founAssociated only with IgA and is found almost exclusively in body secretid almost exclusively in body secretionsons

Enzymatic digestion products of IgEnzymatic digestion products of Ig

Be most susceptible to cleavage near Be most susceptible to cleavage near the hinge regionthe hinge region• Enzyme papainEnzyme papain• Enzyme pepsinEnzyme pepsin

Enzyme papainEnzyme papain

Splits an Ig into 3 fragments of Splits an Ig into 3 fragments of roughly similar sizeroughly similar size

Fab regionFab region: each Fab is monovalent with r: each Fab is monovalent with respect to Ag-binding activityespect to Ag-binding activity

Fc regionFc region: : be recognized by the Fc receptors found be recognized by the Fc receptors found

on many types of cellson many types of cellsDetermine most of the secondary biologiDetermine most of the secondary biologi

c properties of Igc properties of Ig

Enzyme pepsinEnzyme pepsin

Splits an Ig into:Splits an Ig into:A single large fragment ----F(ab)’2 A single large fragment ----F(ab)’2

fragmentfragmentPFc’PFc’

F(ab)’2 fragment:F(ab)’2 fragment: • roughly corresponds to 2 disulfidelinked roughly corresponds to 2 disulfidelinked

Fab fragmentsFab fragments• Has divalent Ag-binding activityHas divalent Ag-binding activity

PFc’:PFc’:• Extensively degradedExtensively degraded• Usually does not survive as an intact fragUsually does not survive as an intact frag

mentment

Hydrolytic fragment of IgHydrolytic fragment of Ig

2 Fab + 1Fc1 F(ab’)2 +pFc’

(papain)(pepsin)

Chapter 2Chapter 2 Ig classes and Ig classes and subclassessubclasses

Ig classes, subclasses and typesIg classes, subclasses and types

• Classes and subclasses are Classes and subclasses are determined on the basis of CHdetermined on the basis of CH

• According to CH sequences and According to CH sequences and their physical and biologic their physical and biologic propertiesproperties

ClassesClasses

• Humans express five different classHumans express five different classes of Ig heavy chainses of Ig heavy chains

• All of the heavy chains in any given IAll of the heavy chains in any given Ig are identicalg are identical

The classes of heavy chainsThe classes of heavy chains

γ μ α δ εγ μ α δ ε

The classes of Ig containing these CHThe classes of Ig containing these CH

IgG IgM IgA IgD IgEIgG IgM IgA IgD IgE

The The μ and ε contain 5 CHμ and ε contain 5 CH

The γ, α and δ contain 4 CHThe γ, α and δ contain 4 CH

Subclasses or subtypesSubclasses or subtypes

• IgG1, IgG2, IgG3, IgG4IgG1, IgG2, IgG3, IgG4

• IgA1, IgA2IgA1, IgA2

All light chains can be classified into 2 All light chains can be classified into 2 types according to CL sequencetypes according to CL sequence• Kappa (κ)Kappa (κ)• lambda (λ)lambda (λ)

No known functional differences exisNo known functional differences exist between two typst between two typs

An Ab molecule has either 2 κ or 2 λ liAn Ab molecule has either 2 κ or 2 λ light chains, but never one of eachght chains, but never one of each

Isotypes, allotypes and idiotypes Isotypes, allotypes and idiotypes of Igof Ig• Ig are proteinsIg are proteins• Ig can be immunogenicIg can be immunogenic

– Isotypic----isotypeIsotypic----isotype– Allotypic----allotypeAllotypic----allotype– Idiotypic----idiotypeIdiotypic----idiotype

isotypesisotypesThe antigenic features of a class of IThe antigenic features of a class of I

g Hg HThe genes for isotypic variantThe genes for isotypic variant

s are present in all healthy s are present in all healthy membersmembers

isotypeisotype

allotypesallotypesVary among individuals within Vary among individuals within

a species and are under a species and are under genetic controlgenetic control

allotypesallotypes

idiotypesidiotypes

• The unique V-region amino acid seqThe unique V-region amino acid sequences of the homogeneous Ig moleuences of the homogeneous Ig molecules produced by a single cules produced by a single B cell cloneB cell clone

idiotypeidiotype

Membrane and secreted IgMembrane and secreted Ig• membrane Ig, mIg : expressen on thmembrane Ig, mIg : expressen on th

e surfaces of resting B lymphocytes, e surfaces of resting B lymphocytes, as receptoras receptor

• secreted Ig, sIg : secreted Ig, sIg :

as antibody(Ab)as antibody(Ab)

Chapter 3Chapter 3 biological activities biological activities

of Igof Ig

Activities of variable regionsActivities of variable regions

Bind to Ag in vivo or in vitro---resistant to infection by:

Opsonizing (coating) organismsBind to viruses, prevent invationNeutralize toxins of microorganisms

返回

Neutralization By Antitoxin Antibodies

Neutralization By Antiviral Antibodies

Bacterial ‘Neutralization’ By Ab

Activities of constant regionsActivities of constant regions

• Complement fixationComplement fixation• OpsonizationOpsonization• ADCCADCC

Complement fixationComplement fixation

Ag-bound IgM and IgG can initiate tAg-bound IgM and IgG can initiate the classical pathway of complemenhe classical pathway of complement activationt activation

Activated complement mediated lysActivated complement mediated lysis of the pathogen (or other cell)is of the pathogen (or other cell)

Opsonization Opsonization IgG is an opsonizing Ab Fc confers the opsonizing propertyMany phagocytic cells bear receptor

s of the Fc region of IgG• Macrophages• Polymorphonuclear phagocytes• phagocyte

调理作用OpsonizationOpsonization

ADCCADCC

FcRIII CD16

Antibody Marks Target Cells For NK Cell Attack (ADCC)

Chapter 4Chapter 4 biological properties anbiological properties an

d functions of Ig classesd functions of Ig classes

IgGIgG

About 75% of the total serum Ig in normaAbout 75% of the total serum Ig in normal adultsl adults

The most abundant Ab produced during sThe most abundant Ab produced during secondary IRecondary IR

The only class of Ig that can cross the plaThe only class of Ig that can cross the placentacenta

Be capable of fixing serum complementBe capable of fixing serum complement Bind the Fc receptorsBind the Fc receptors

IgAIgA• In secretions: SIgA ---- dimerIn secretions: SIgA ---- dimer

• On B cell surfaces On B cell surfaces monomermonomer• In the bloodIn the blood

The predominant Ig produced by B cThe predominant Ig produced by B cells in submucosal lymphoid tissuesells in submucosal lymphoid tissues

The most abundant Ab class found iThe most abundant Ab class found in secretions (milk, bronchial secretin secretions (milk, bronchial secretions, intestinal mucus)ons, intestinal mucus)

IgMIgM

Pentamer in serumPentamer in serumPredominate in early primary immunPredominate in early primary immun

e responsese responsesThe most efficient complement fixing The most efficient complement fixing

IgIgExpressed on the surfaces of B cells, Expressed on the surfaces of B cells,

particularly naïve B cellsparticularly naïve B cells

Can be synthesized during the late eCan be synthesized during the late embryo stagembryo stage

The first Ig produced in primary immThe first Ig produced in primary immune responsesune responses

Natural blood group antibodyNatural blood group antibody

IgDIgD

The physiologic function of IgD is unThe physiologic function of IgD is unknownknown

Rarely secreted in significant amouRarely secreted in significant amounts in bloodnts in blood

Commonly found on the surfaces of Commonly found on the surfaces of B cellsB cells

mIgM mIgD

成熟 B 细胞 活化 B 细胞未成熟 B 细胞

mIgD can bind Ag and transmit signmIgD can bind Ag and transmit signals to the cell interiorals to the cell interior

When B cells containing mIgD becoWhen B cells containing mIgD become activated, mIgD expression ceasme activated, mIgD expression ceaseses

IgEIgERepresents only a minute fraction oRepresents only a minute fraction o

f all serum Abf all serum AbHas central involvement in allergic Has central involvement in allergic

disordersdisorders In monomeric formIn monomeric form

FcRI

IgE Antibody Binds To Mast Cells & Basophils To Arm Them For Mediator Release

Chapter 5Chapter 5

Ab technologyAb technology

Poly-clonal abtibody, PcAb The antiserum from an animal The antiserum from an animal immunized against an immunogen is aimmunized against an immunogen is a complex mixture of structurally diverse complex mixture of structurally diverse Abs recognizing multiple epitopes on Abs recognizing multiple epitopes on the immunogen, and its composition the immunogen, and its composition fluctuates unpredictably over the life of fluctuates unpredictably over the life of the animal the animal

带有多个表位

抗原

针对不同表位的 B 细胞

克隆增殖

多种抗体的混合物

（多克隆抗体）

刺激小鼠

Mono-clonal antibody,McAbMono-clonal antibody,McAb