分析03.上課定稿igenglish

1、Antibodies(Immunoglobulins)1. Introduction2. Structure3. Functions4. Overview of the 5 classes of Igs5. Preparation of antibodiesAntibody response1. IntroductionAntibodies:A group of glycoproteins produced by B cells, which bind antigens with high specificity, often termed “immunoglobulins”.1-1. Dis

2、tribution 1-2. Heterogeneity1-1. Distribution of antibodies Serum and tissue fluidsMediating the humoral immune response Surface of B cellsActing as B cell antigen receptor (BCR)Surface and secreted antibodies1-2. Heterogeneity of antibodies to fractions of normal serum many classes and types specif

3、ic for an antigen epitopeElectrophoretic distribution of the major human immunoglobulins5 classes of human Absdepending on the structure of the heavy chainAntibody binds Antigen specifically2. The structure of antibodies2-1. Basic structure of antibody2-2. Antibody fragmengts2-3. Structure of 5 clas

4、ses of antibodies2-1. Basic Structure of Antibody (antibody unit) Basic structure of Ab is a 4-chain unit Variable region and constant region Immunoglobullin domains2 identical light chains (L chain)2 identical heavy chains (H chain)The L and H chains are linked by disulphide bonds, forming a Y-shap

5、ed molecule 2 kinds of L chain: , , forming 2 types of antibodies 5 kinds of H chain: , , , and , forming 5 classes of antibodies.(IgG, IgM, IgA, IgD, IgE)Basic structure of Ab is a 4-chain unitBasic structure of AbHL V(variable) regionThe amino-terminal(N) part of the chain, consisting of 100110 am

6、ino acids, are found to vary among different Abs.V region can be divided into hypervariable region (complementarity determining region CDR, idiotypic determinant) and framework region C(constant ) regionThe carboxyl-terminal(C) part of the molecule, called the constant region,which is identical for

7、antibodies of all specificities within a particular class. Variable region and constant region All chains have one variable and one constant region.NCDomains of Antibody A disulphide bond enclosed peptide loop L chain: VL and CL H chain: VH, CH1, CH2. CH3 (IgG, IgA, IgD; IgM and IgE have got an extr

8、a CH4)The domains of IgG1NC VL and VH: binding an antigenic epitope specifically CH2: binding the C1q of complement. CH3 (or CH4 of IgM and IgE): binding to the Fc rececptor on effector cells.Some functions of the domainsHinge region and the flexibility of antibody2-2. Antibody fragmentsAntibody can

9、 be proteolytically cleaved into fragments Fab fragment (fragment that binds antigen) antigen binding part of antigody Fc fragment (fragment that crystallized) effector part of antibodyProteolytic fragments of antibodyAB2-3. Structure of 5 classes of antibodies IgG, IgE, IgD and serum IgA are of a t

10、ypical four-chain structure (monomer). IgM is a pentamer of the basic four-chain units (composed of 5 four-chain units), these units are linked by a peptide called J chain. Secretory IgA (sIgA) is a dimer of two basic four chain units, these units are linked by the J chain, A peptide called secretor

11、y component is also needed for sIgA traverses epithelial cell layers.Structure of secretory IgAStructure of IgM3. Functions of antibodiesAntibodies are bifunctional molecules3-1 V regionbinding to antigen (epitope) specifically3-2 C regioneffector functions Antibody on B cell serface (BCR) B cells r

12、ecognize antigen (epitope) specifically Antibody in serum and intrastitial fluid Neutralizing bacterial toxin, preventing viral attachment to host cells 3-1. Functions of V regionVariable regions account for unique antigen binding specificities Complement activation (classical pathway) Binding to Fc

13、 receptors on effector cells - ADCC - Opsonization - Type I hypersensitivity Other functions - placental transfer, epithelial transfer3-2. Functions of C regionConstant regions mediate different effector functionsReceptors for Igs (Fc receptors, FcR) Membrane-bound proteins expressed on the surface

14、of a variety of effector cells which bind to the Fc region of antibodies, mediating different antibody functions.eg. FcR is exressed on phagoytes (PMNs, macrophages and eosinophils), NK cells.FcR is expressed on mast cells and basophils.Antibody dependent cellular cytotoxicity (ADCC)PMNs, monocytes,

15、 macrophages, NK cells, eosinophils can kill antibody coated target cells directly.Fab of Ab binds the antigen of the target cell, and the Fc of Ab binds to the FcR on the effector cells, mediating the lysis of the target cells.OpsonizationProcess of making a microbe easier to phagocytose. Antibody

16、coating the microbe will facilitate the phagocytes to engulf it.Fab of Ab binds the antigen of the microbe, and the Fc of Ab binds to the FcR on the phagocytes, facilitating the phagocytosis of the antigenIgE mediating type I hypersensitivityThe Fc of IgE binds to FcR on mast cells and basophils, an

17、d the Fab binds antigen, mediating the hypersensitivity4. Overview of the 5 classes of antibodies IgG IgM IgA IgD IgEIgG The major Ig in serum and tissue fluid, 70-75% of total Igs IgG is a monomer of a 4-chain unit The predominant Ig produced in secondary response The most important Ig involved in

18、elimination of pathogens and mediation of inflammation (complement activation, opsonization, ADCC) Placental transfer, IgG is the only Ig class cross the placentaIgM 10% of the total Ig pool, in serum (intravascular) A pentamer of 4-chain units, with the largest molecular weight(macroglobulin) The p

19、redominant and earliest produced Ig in primary response. IgMnew infectionIn the process of individual development, IgM is the earliest produced Ig(In the advanced stage of embryo development). IgM in umbilical cord bloodintrauterine infection Associated with immune response to antigenic complex, blo

20、od-borne infectious organisms (A potent activator of complement)IgA 15-20% of the total Ig, 2 forms of IgA (serum IgA and secretory IgA) Serum IgA is a monomer of 4-chain unit, secretory IgA is a dimer of 4 chain units The secretory IgA is the predominant Ig in seromucous secretions (saliva, colostrum, milk, tracheobrochial and genitourinary secretions) involved in seromucous defense (sIgA)IgD 1% of the serum Ig, but a major Ig on B cells (mIgD) Mature B cells express mIgM and mIgD. Immature B cells only express mI