医学免疫学英文版教学课件：Chapter 5 Immunoglobulin

1、Chapter 5 Immunoglobulin(gamma) globulins and AntibodyPart I. IntroductionAntibody (Ab) : Ab is a kind of globulin that can specifically combine with a corresponding Ag. It is produced by B cells in response to the stimulation of Ag.Immunoglobulin( Ig): All globulins that posses the activation of Ab

2、 or show a similar structure to Ab. Molecular Structure of Ig 1. Basic structure (four polypeptide chains) Ig is composed of four polypeptide chains joined by S-S bonds. inter-chain disulfide bonds (S-S) intra-chain disulfide bonds (S-S) It show “T” or “Y” shape.(1) H and L chain. heavy chains (H):

3、450 550aa, 5075KD. light chains (L) 214 aa, 25KD Two terminal ends for each peptide “N” terminal end “C” terminal end C-terminal endN-terminal end (2) classes and types of Ig According to the differences of H chains ( amino acid composition , sequence) Igs can be divided into 5 classes Five classes

4、of H Chain: Five classes of Ig : IgG IgA IgM IgD IgE Subclass: IgG1-4 Two types of L chain : , : 20:1 (in mice) 2:1 (in human) Subtype: l14(3) Two regions of each peptideConstant region ( C )2) Variable region (V)Constant region ( C )CH: 3/4 or 4/5 of H chain of the c end CL: 1/2 of L chain 2) Varia

5、ble region (V) Hypervariable region(HVR) There are three highly diversity stretches within the V region ,they are called HVR.Framework region (FR) Complementarity-determining region (CDR) Three HVR is brought together to form antigen-binding surface, so HVR is also called CDR. - H: CDR1, CDR2,CDR3 -

6、 L: CDR1, CDR2,CDR3 Idiotype of IgImmunoglobins produced by different B cell clones possess unique structure in hypervariable region(HVR) , this unique structure of Ig is called idiotype of Ig. In fact，HVR, CDR and idiotype refer to the same site of Ig. (4) hinge regionhinge region: between CH1 and

7、CH2 (rich in proline ,easy to flex ) flexible and suitable for CDR of Ig to bind with antigenic determinants. (5) Other structures of Ig Joining chain(J ) Secretory piece( SP):Joining chain(J ) : produced by plasma cells Functions: linker to compose dimer ( Ig A) 、pentamer(IgM) or polymerSecretory p

8、iece( SP): . produced by mucosa epithelial cells . exist in secretory IgA (SIg A) . Functions: protect SIgA , resist proteolysis in extra secretory liquid.(1) Domain : Polypeptide chains of Ig are folded by intra-chain s-s bond to form globular shape in every 110aa regions which is called a domain.

9、immunoglobulin domain2. Domains of Ig(2) Domains of IgL chain: VL, CLH chain(45 ) VH, CH1, CH2, CH3/CH4( IgM,IgE)(3) function of each domain VH , VL: antigen-binding siteCH, CL: allogeneic markerCH2: complement-fixing site CH3/CH4: cell-binding site CH(IgG):placenta-crossing siteIV. Hydrolysis fragm

10、ents of Ig and their functions1) Digested with papain2) Digested with pepsin1) Digested by papainPosition : Near the inter-chains S-S bonds of H (from the N end) Fragments: 2Fab ,Fc Function: Fab fragment of Ag binding Fc 1) fix complement 2) crossing the placenta 3) band to FcR in different cells 2

11、) digested by pepsinPosition: near the S-S bond of H inter-chains (from the C end)Fragments and function : F(ab)2 bind antigen (2 values) pFc no function Significance Be useful in elucidating structure and functional relationships of Igs.Decrease the immunogenicity of Ig for clinical treatment.Secti

12、on II Immunogenicity of Ig isotype allotype idiotype Isotype Allotype Idiotype I. Isotype of Ig The epitope of Ig existing in all healthy individuals of a species is called as isotype This is a kind of species specificity which exists in C region of immunoglobulin, including class, subclass, type, s

13、ubtype II. Allotype: The property of a group of antibody molecules defined by their sharing a particular antigenic determinant found on the antibodies of some individuals but not others of a species This is a kind of individual specificity within a species which exists in C region of immunoglobulin

14、III. Idiotype of Ig Immunoglobins produced by different B cell clones possess unique structure in hypervariable region(HVR) , this unique structure of Ig is called idiotype of Ig.Section III Biological activities of antibodyII. The function of C region(2)ADCC (antibody-dependent cell-mediated cytoto

15、xicity) (3)Opsonization(4)Activation of complement(5)Transportation through placenta and mucous(6)Mediating hypersensitivityI. Function of V region specifically combined with antigen(1)Neutralization(1)Neutralization: * neutralize microbial toxins and animal venoms * prevent viruses and bacteria fro

16、m infecting cells- antibody responses can be long-lasting can exist for years after the initial encounter or vaccination (e.g. 10 years for tetanus toxoid vaccination)(2) ADCC(antibody-dependent cell-mediated cytotoxicity) ADCC is a mechanism whereby an effector immune cell actively lyses a target c

17、ell that has been bound by specific antibodies. Antibody-dependent cellular cytotoxicity (ADCC)(3)Opsonization macrophageFc receptors on phagocytes trigger the uptake of antibody-coated bacteria.As a result, this action enhance phagocytosis of the bacterium.Complement proteins bind to antibodies.(4)

18、Activation of complement Complement can be activated to directly lyse bacteria by the presence of antibodies bound to the bacteria.The biological functions of complement(5) Transportation through mucous and placentaTransport of IgA through epitheliumMaternal IgG is transported across the placenta to

19、 the fetus(6) Mediating hypersensitivityThe effector mechanisms of humoral immunityPart IV Five clasess of Ig and their functions IgG IgA IgM IgD IgEI. IgGMonomerFour subclasses : IgG1, IgG2, IgG3, IgG4IgG 1.Highest concentration in serum(75% of total Ig)2.Unique Ig that can pass through placenta.3.

20、half-life is longer( 20-23 days)4. combine with the Fc receptor(FcR)5. Fixes complement 6.The individual starts to produce Ig G at 2-3 month after birth and reach the level of adult at 5 years old II. IgAStructureSerum - monomerSecretions (sIgA)Dimer J chainSecretory componentJ ChainSecretory PieceP

21、roperties:1. Concentration in serum is high(10-20%)2. Major secretary Ig (Mucosal or Local Immunity)Tears, saliva, gastric and pulmonary secretions exist in almost all humoral fluid3. Binds to Fc receptors on some cells4. Activate the complement by alternative pathway 5. Be produced at 4 month after

22、 birthIII. IgMStructurePentamer (90KD)Extra domain (CH4)J chainCH4J ChainIII. IgM1.Highest MW：pentamer（90KD）Valences：10 valences in theory2.Half-life is shorter(45 days)3.The first Ig to be synthesizedBe firstly produced during fetusAppear firstly in the early stage after infection the first mIg of

23、the B cells-(BCR)4.Fixes complement5. natural Ab for blood-type antigen 6. auto-antibody :rheumatoid factor(RF)Rheumatoid arthritis ,RAJoint deformityIV. IgDStructureMonomerTail pieceTail Piece1. The concentration in serum is low and sensitive to proteinase 2. Act as the antigen receptor on B cells

24、(mIgD): Regulate the differentiation of B cellsimmature B cells mature B cells activated/memory B cellsmIgM mIgD V . IgE1.Concerntration of IgE in serum is the lowest in normal individual, but is very high in some patients2.Related to type I persensitivity degranulation IgEFceRIallergininflammationPart IV. Preparation of Ab Polyclonal AbMonoclonal AbGene engineering Ab1.Polyclonal Ab A mixture of Ab molecules of different specificities and affinities against different epitopesgenerated in a natural response or after immuniza