Section 4

Outline Outline  Introduction  Structure of Immunoglobulin  Function of Immunoglobulin  Characteristics and Functions of 5 Ig Classes of 5 Ig Classes  Monoclonal Antibody (conception of Ab and Ig)

An enormous family of related, but nonidentical glycoproteinsAn enormous family of related, but nonidentical glycoproteins

Expression of Ig membrane Ig, mIg : expressen on the surfaces of resting B lymphocytes, as receptormembrane Ig, mIg : expressen on the surfaces of resting B lymphocytes, as receptor secreted Ig, sIg : as antibody(Ab)secreted Ig, sIg : as antibody(Ab)

Major functions of Ig Binding AgBinding Ag Secondary biologic activitiesSecondary biologic activities –Opsonins –Activate the complement cascade –Cross the placental barrier

Chapter 1 organization of organization of immunoglobulins immunoglobulins

Molecular structure of Ig An enormous family of related, but nonidentical glycoproteinsAn enormous family of related, but nonidentical glycoproteins Ab are bifuctional molecules:Ab are bifuctional molecules: bind specifically to Ag bind specifically to Ag secondary phenomena secondary phenomena

The four-chain basic unit Symmetrical structure  Two Heavy (H) chains, about twice as large as the L chain  Two Light (L) chains, the smaller  Covalent interchain disulfide bridges

(Ag-biding site) L chain H chain (,,,,)(,,,,) V region C region  H2L2 H2L2 H2L2 H2L2  hinge region  2 regions κ/λ (2:1) Monomer of Ig

domains  Both H and L chains contain folded globular domains  Each domains has a single intrachain disulfide bond  L chains always contain two domains  H chains contain either 4 or 5 domains

Immunoglobulin variable regions and constant regions variable region (V region) The sequences of the N-terminal The sequences of the N-terminal domain in a heavy- or light-chain polypeptide are variable – V H – V L

constant region (C region) The sequences of the other domains remain relatively constantThe sequences of the other domains remain relatively constant –C H –C L L-chain polypeptides contain only a C L domainL-chain polypeptides contain only a C L domain H-chain polypeptides contain 3 or more C H domainsH-chain polypeptides contain 3 or more C H domains

（二）可变区和恒定区

 The V H is juxtaposed with the V L to form an Ag-binding site  Each monomer of Ig contains two Ag-binding sites

Hinge region  Located between the CH1 and CH2 of the H chains in most Ig  Permits flexibility between the two Fab arms of Ab  Allows the two arms to open and close to accommodate binding to two epitopes

Flexibility of immunoglobulins

Hypervariable regions The V regions contain two regions:  Framework regions----relatively invariant  Hypervariable regions----extremely variant Complementarity-determining regions (CDRs)

Complementarity-determining regions (CDRs) Each VH and VL contains 3 CDRs

Antigen-binding Can Take Many Forms – Best Method

J chain  IgM and IgA generally exist as polymers  Joint  Joint monomers to form polymers n(H 2 L 2 ) n(monomer) n(H 2 L 2 ) n(monomer) J chain (H 2 L 2 )n J polymer polymer J chain and secretary component

Secretory component Associated only with IgA and is found almost exclusively in body secretionsAssociated only with IgA and is found almost exclusively in body secretions

Enzymatic digestion products of Ig Be most susceptible to cleavage near Be most susceptible to cleavage near the hinge region Enzyme papainEnzyme papain Enzyme pepsinEnzyme pepsin

Enzyme papain Splits an Ig into 3 fragments of Splits an Ig into 3 fragments of roughly similar size

Fab region: each Fab is monovalent with respect to Ag-binding activity Fab region: each Fab is monovalent with respect to Ag-binding activity Fc region:  be recognized by the Fc receptors found on many types of cells  Determine most of the secondary biologic properties of Ig

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

F(ab) ’ 2 fragment: roughly corresponds to 2 disulfidelinked Fab fragmentsroughly corresponds to 2 disulfidelinked Fab fragments Has divalent Ag-binding activityHas divalent Ag-binding activity PFc ’ : Extensively degradedExtensively degraded Usually does not survive as an intact fragmentUsually does not survive as an intact fragment

Hydrolytic fragment of Ig 2 Fab + 1Fc 1 F(ab ’ ) 2 +pFc’ (papain)(pepsin)

Chapter 2 Ig classes and Ig classes and subclasses subclasses

Ig classes, subclasses and types Classes and subclasses are determined on the basis of CHClasses and subclasses are determined on the basis of CH According to CH sequences and their physical and biologic propertiesAccording to CH sequences and their physical and biologic properties

Classes Humans express five different classes of Ig heavy chainsHumans express five different classes of Ig heavy chains All of the heavy chains in any given Ig are identicalAll of the heavy chains in any given Ig are identical

The classes of heavy chains γ μ α δ ε γ μ α δ ε The classes of Ig containing these CH IgG IgM IgA IgD IgE IgG IgM IgA IgD IgE The μ and ε contain 5 CH The γ, α and δ contain 4 CH

Subclasses or subtypes IgG1, IgG2, IgG3, IgG4IgG1, IgG2, IgG3, IgG4 IgA1, IgA2IgA1, IgA2

CH of all Ab molecules of one CH of all Ab molecules of one isotype of subtypes have essentially isotype of subtypes have essentially the same amino acid sequence the same amino acid sequence

 All light chains can be classified into 2 types according to CL sequence Kappa (κ)Kappa (κ) lambda (λ)lambda (λ)  No known functional differences exist between two typs  An Ab molecule has either 2 κ or 2 λ light chains, but never one of each

Isotypes, allotypes and idiotypes of Ig Ig are proteinsIg are proteins Ig can be immunogenicIg can be immunogenic –Isotypic----isotype –Allotypic----allotype –Idiotypic----idiotype

isotypes  The antigenic features of a class of Ig H  The genes for isotypic variants are present in all healthy members isotype

allotypes  Vary among individuals within a species and are under genetic control allotypes

idiotypes The unique V-region amino acid sequences of the homogeneous Ig molecules produced by a single B cell cloneThe unique V-region amino acid sequences of the homogeneous Ig molecules produced by a single B cell clone idiotype

Membrane and secreted Ig membrane Ig, mIg : expressen on the surfaces of resting B lymphocytes, as receptormembrane Ig, mIg : expressen on the surfaces of resting B lymphocytes, as receptor secreted Ig, sIg :secreted Ig, sIg : as antibody(Ab) as antibody(Ab)

Chapter 3 biological activities of Ig biological activities of Ig

Activities of variable regions Bind to Ag in vivo or in vitro---resistant to infection by:  Opsonizing (coating) organisms  Bind to viruses, prevent invation  Neutralize toxins of microorganisms

返回

Neutralization By Antitoxin Antibodies

Neutralization By Antiviral Antibodies

Bacterial ‘Neutralization’ By Ab

Activities of constant regions Complement fixationComplement fixation OpsonizationOpsonization ADCCADCC

Complement fixation  Ag-bound IgM and IgG can initiate the classical pathway of complement activation  Activated complement mediated lysis of the pathogen (or other cell)

Opsonization  IgG is an opsonizing Ab  Fc confers the opsonizing property  Many phagocytic cells bear receptors of the Fc region of IgG Macrophages Polymorphonuclear phagocytes phagocyte

调理作用 Opsonization

ADCC

Fc  RIII CD16 Antibody Marks Target Cells For NK Cell Attack (ADCC)

Chapter 4 biological properties and functions of Ig classes biological properties and functions of Ig classes

IgG  About 75% of the total serum Ig in normal adults  The most abundant Ab produced during secondary IR  The only class of Ig that can cross the placenta  Be capable of fixing serum complement  Bind the Fc receptors

IgA In secretions: SIgA ---- dimerIn secretions: SIgA ---- dimer On B cell surfacesOn B cell surfaces monomer monomer In the bloodIn the blood

 The predominant Ig produced by B cells in submucosal lymphoid tissues  The most abundant Ab class found in secretions (milk, bronchial secretions, intestinal mucus)

IgM  Pentamer in serum  Predominate in early primary immune responses  The most efficient complement fixing Ig  Expressed on the surfaces of B cells, particularly na ï ve B cells

 Can be synthesized during the late embryo stage  The first Ig produced in primary immune responses  Natural blood group antibody

IgD  The physiologic function of IgD is unknown  Rarely secreted in significant amounts in blood  Commonly found on the surfaces of B cells

mIgMmIgD 成熟 B 细胞活化 B 细胞未成熟 B 细胞  mIgD can bind Ag and transmit signals to the cell interior  When B cells containing mIgD become activated, mIgD expression ceases

IgE  Represents only a minute fraction of all serum Ab  Has central involvement in allergic disorders  In monomeric form

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

Chapter 5 Ab technology Ab technology

Poly-clonal abtibody, PcAb The antiserum from an animal The antiserum from an animal immunized against an immunogen is a immunized 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,McAb