Chapter 4 Immunoglobulin,Ig

discovery of Ig Antibody,Ab antibodies is first discovered by Behring and Kitasato in 1890, antitoxin----(diphtheria exotoxin) capable of neutralizing the effects of toxin. r globulin,immunoglobulin Immune serum---antiserum

Emial von Behring and Hidesaburo Kitasato were the first to show that antibody elicited in one animal can be transferred to another by injecting antibody with serum from the first von Behring

antibody，Ab. Ag→B cell→plasma cell→ antibody Antibody---A protein that are produced by B cells that have differentiated to become plasma cells and that have the ability to combine with the antigen which stimulated its production.

Antibodies (Ab) are circulating proteins that specifically bind to antigen Each antibody has a specificity different from the others antibodies Each B cell makes ONE and only ONE type of antibody--clonal selection

gamma－globulin

immunoglobulin: globulins with antibody activity or its chemical structure is similar with Ab. Ig----structural and chemical Ab---biological and functional All Abs are Igs, but not all Igs are Abs. The terms Ig and antibody are used interchangeably. secreted Ig，sIg——Humoral immunoresponse membrane Ig，mIg——BCR

§ 1 Structure of Immunoglobulin Ig are glycoproteins.Each person is capable of producing at least 108 different antibody molecules.every immunoglobulin molecule is made up of two different types of polypepetides. The larger,heavy(H2)chins are roughly twice as large as The smaller,light(L2) chains.(H2L2)

1、Basic structure of Ig four-polypeptide chains unit， held together by a number of interchain disulfide bonds,form a Y-shaped construction，called Ig monomer，the basic unit. amino-terminal---N terminal carboxy-terminal---C terminal

（1）heavy chain and light chain 1)、heavy chain，H mW 50~75kD，each chain is composed 450~550 amino acids.heavy chains contain either four or five domains,each of which is 100-110 amino acides long and contain a single intrachain disulfide bond.

Ig IgM IgG IgA IgD IgE μ γ α ε δ Ab

2)、light chain，L mW 25kD，214aa.  or  type， 1~  4 (subtype)

1)、variable region(VH,VL) （2）variable region and constant region 1)、variable region(VH,VL) hypervariable region，HVR （complementarity-determining region，CDR）， HVR1（CDR1）、HVR2（CDR2）and HVR3（CDR3）-- antigen-binding site， framework region，FR. VH and VL contain four FR

2)、constant region(CH,CL) CH: CH1、CH2、CH3、CH4：The chain C regions interact with other molecules and cell of the immune system and therefore mediate most of the biologic functions of antibodies.

3)、hinge region hinge region located between the CH1 and CH2 domains of the H chains.it is maded up predominantly of cysteine and proline residues.It provides an important flexibility of Ig

4).domain

Function region： ①VH and VLis antigen-binding site. ②CH1 and CLgene mark ③CH2 can bind complement and Penetrating Placenta ④CH3 is binding cell FcR。

Ig β sheet--- β sandwich, immunoglobulin folding Conception of immunoglobulin superfamily,IgSF It is membrane surface molecule that structure and gene similar to immunoglobulin and have recognized function is called immunoglobulin superfamily,IgSF

2、other components of immunoglobulin （1）joining chain,J chain IgM sIgA generally include a single additional polypeptide is called the J chain.It is synthesized by all plasma cell that secrete polymeric Ig.Its function seems to be able to facilitate proper polymerization

（2）secretory piece，SP It is a single glycopeptide with a peptide mW 7000,and is synthesized by mucosal epithelial cell,its function is resistant to proteolytic digestion

3、 enzymatic digestion fragmentation of Ig （1）papain Ig molecules are cleaved by the enzymes papain allows separation of two fragment antigen binding，Fab2 and One fragment crystallizable，Fc1

（2）pepsin Pepsin generates a single bivalent antigen-binding fragment, (Fab ’ ）2 and pFc’ Refine tetanus antitoxin with pepsin

§ 2 heterogeneity( diversity) of immunoglobulin classes and type in Ig

divided into subclasses basis （1）class :Ig can be divided into classes on basis of difference the structure in CH .IgG、IgA、IgM、IgD and IgE。 （2）subclass:IgG and IgA can be divided into subclasses basis of difference the antigenic of CH and S-S: IgGl~IgG4, IgA—IgAl and IgA2

（4) subtype: leucine-- OZ（十）arginine-- OZ（一）in  190 （3） type : Ig can be divided into  and  basis of difference the structure in CL （4) subtype: leucine-- OZ（十）arginine-- OZ（一）in  190

3 endogenous—diversity of Ig epitope 2 exogenous ——diversity of antigen 3 endogenous—diversity of Ig epitope serologic type of immunoglobulin ： （1）Isotype:CH （2）allotype:CH s-s （3）idiotype，Id: Fab anti-idiotype antibody，AId

§ 3 Function of Immunoglobulin

1、function of V region Specifically binding with antigen

Virus-blocking Antibodies

2、function of C region （1）activating complement （2）binding to Fc receptor 1)、opsonization

2)、antibody-dependent cell-mediated cytotoxicity，ADCC

3)、 mediating type I hypersensitivity

（3） Penetrating Placenta and Mucous Membrane

§ 4 Biological properties and function of every immunoglobulin

1、IgG Monomer sedimentation coefficient---7s molecular weight---150,000 begin to synthesize 3 months after birth, reach adult level in 3～5 y. 1) half-life long---23d 2) percent of total serum Ig---75% ～80% subclass---IgG1, IgG2, IgG3, IgG4 3) main antibody when secondary immune response

4) passage through the placenta 5) major anti-infection antibody agglutination and formation of precipitate 4) passage through the placenta 5) major anti-infection antibody Opsonization antibody-dependent cell-mediated cytotoxicity activation of complement neutralization of toxin immobilization of bacteria neutralization of viruses Binding with staphylococcus protein A，SPA，for diagnosis Taking part in II、III type hypersensitivity

2、IgM 1) Molecular mass of approximately 900KD,macroglobulin 2) In early primary production 3) BCR 4) Most efficient complement-fixing

3、IgA On B cell surfaces or in the blood , IgA exists as monomer comprising only one four-chain unit. Secretion IgA,sIgA are dimers .it can be Local immune

4、IgD mIgD---BCR,auto-immune disease The physiologic function of IgD is unknown

5、IgE 1)Allergic response:mast cell and the basphil—carry 104 unique FcεR, 2)high –affinity :Fc ε R that is specific for IgE antibodies. 3) low concentration :IgE(10-7) in blood and tissue fluids.

§ 5 preparation of artificial antibody 1.poly-clonal antibody，pAb pAb is a complex mixture of antibodies recognizing multiple epitopes on the immunogen –Antiserum--antitoxin

2. monoclonal antibody，mAb (1).Conception: mAb is a kind of antibody that is specific for one type of epitope and is produced by single B cell clone.It have high-specialang sensitivity It is prepared by hybridoma technique and was described by Georges Kohler and Cesar Milstein in 1975 (2). preparation : Immune B cell+myeloma→hybridoma →clonal →positive clones→expand →get antibody

(3)Applications mAb have found numerous applications in diagnosis,as tools for clinical treatment,especially in cancer.for example, radioactively labeled mAb that recognize tumor. Treatment –immunotoxin Research application

3.Genetic engineered antibody

Chimeric antibody Reshaped antibody Bispecific antibody Phage antibody Small molecule antibody(Fab; Fv ; single domain Ab;single chain Ab)