Medical Immunology

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Medical
Immunology

• Chapter 5 Immunoglobulin
• 
  zhang lining
• 
  Immunology Institute Medical School
• 
  Shandong University

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Antibodya type of glycoprotein molecule,
produced by B cells that bind antigens often with
a high degree of specificity The basic
structural unit of antibody is composed of two
identical heavy chains and two identical light
chains
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• Immunoglobulin( Ig)
• It refers to all globulins that possess the
  bioactivity of Ab or a similar structure to Ab
• Therefore, all Abs belong to Igs, but not all
  Igs possess the functions of Abs

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?-globulin
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Section I. Molecular Structure of
Immunoglobulin
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I. Basic structure of Ig

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• Four polypeptide chains
• 2 identical heavy chains
• 2 identical light chains
• The 4 polypeptide chains
• are joined by S-S bonds.
• inter-chain disulfide bonds (S-S)
• intra-chain disulfide bonds (S-S)
• Two terminal ends
• N terminal end
• C terminal end
• Two regions
• variable regions
• constant region

N-terminal end
C-terminal end
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1.Heavy and Light chain

• . Heavy chains (H)
• 450 550aa,
• 5075KD
• . Light chains (L)
• 214 aa, 25KD

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Classes and types of Ig

• According to the differences of H chains(
  amino acid composition , sequence), H chains can
  be divided into
• 5 classes and their constituted Ig can be
  classified into 5 classes.
• Five classes of H Chain ? ? ? ? ?
• Five classes of Ig IgG IgA IgM IgD IgE

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• According to the differences of L chains
• ( amino acid composition , sequence),
• L chains can be divided into 2 types
  and their constituted Igs can be typed into 2
  types , ? and ?
• ? ? 201 (in mice)
• 21 (in human)

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2. variable regions and constant regions

• Variable region (V)
• ½ of L chain or ¼ of H chain from N end
• 2) Constant region ( C )
• ½ of L chain or 3/4 of H chain from C end

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• (1) Variable region (V)
• Hypervariable region (HVR)
• Framework region (FR)

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• Hypervariable region (HVR)
• Most of the sequence differences among
  antibodies are confined to three short stretches
  in the V regions of heavy and light chains are
  called HVR.
• Because these sequences form an
  antigen-binding surface that is complementary to
  the three-dimensional structure of the bound
  antigen, HVR are also called Complementarity
  determining regions, CDRs
• L chain possesses 3 CDRsCDR1, CDR2 and CDR3
• H Chain possesses 3 CDRs CDR1, CDR2 and
  CDR3

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• CDR complementarity determining regions
• The three short stretches in the V regions of
  Ig that contain most of the sequence differences
  among Igs are called CDR because these sequences
  form an antigen-binding surface that is
  complementary to the three-dimensional structure
  of the bound antigen

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L CDR128-35, CDR249-56,
CDR391-98
H
CDR1-29-31 CDR2-49-58 CDR3-95-102
CDR1, CDR2,CDR3
CDR1
CDR3
CDR2
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• Ag-binding sites
• L CDR128-35,
• CDR249-56,
• CDR391-98
  
• H CDR1-29-31
• CDR2-49-58
• CDR3-95-102
• The antigen-binding site binds to epitope of
  antigen

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CDR3
CDR1
CDR2
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Ag-binding sites
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3. Hinge region

• The hinge region is segment of heavy chain
  between the CH1 and CH2 domains.
• Flexibility in this area permits the two
  antigen-binding sites to operate independently.

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II. Other components of Ig

• Joining chain(J )
• Secretory piece( SP)

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Joining chain (J )

• produced by plasma cells
• Functions linker
• Join monomer of Ig to form dimer, or
  polymer
• 
  ( Ig A, IgM)

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Secretory piece( SP)

• . produced by mucosa epithelial cells
• . Bind to dimer of IgA to form secretory IgA
  (SIg A)
• . Functions protect SIgA against proteolysis
  in
• secretory liquid.

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Secretory piece( SP)
J chain
Secreted piece
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Formation of secreted piece
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III. Domains of Ig

Domains the Polypeptide chains of Ig are
folded by intrachain s-s bond into globular shape
in each 110aa regions which is called a domain
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• The domains of L chain 2 domains ,VL and CL
• The domains of L chain 4-5 domains
• 4 domains VH, CH1, CH2, CH3 in IgG, IgD, IgA
• 5 domains VH, CH1, CH2, CH3 CH4 in IgM, IgE

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Function of domains

• VH , VL antigen-binding site
• CH1, CL genetic marker
• 2CH2 complement-fixing site or
• crossing the placenta
• CH3/CH4 cell-binding site
• hinge region flexible and suitable for CDR of
  Ig bond to antigenic determinants.

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IV. Proteolytic fragments of an IgG molecules
and their functions

• Ig digested by papain and pepsin
• .position
• .Fragments
• .Function

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1. Products of IgG digested by papain

• Position
• near the N terminus of S-S bonds of H
  inter-chains
• fragments
• 2Fab (antigen-binding fragment)
• Fc (crystalizable fragment)
• Function
• Fab bind specifically to Ag
• Fc 1) fix complement
• 2) cross the placenta
• 3) bind to FcR on different cells

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• 2. Products of IgG digested by pepsin
• Position
• near the S-S bond of H inter-chains from the
  C end
• Fragments and function
• F(ab)2 bind to antigen (2 values)
• pFc no function

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Significance

•  Elucidating the relationships between the
  structure and function of Igs .
• Decrease the immunogenicity of Ig for clinical
  treatment

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Section II .The features and functions of
different classes Ig
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I . IgG 1. Highest concentration in
serum (75 of total Ig)
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• 2. Four subclasses IgG1, IgG2, IgG3, IgG4

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• 3.Unique Ig that can pass through placenta.
• 4.half-life is longest one among all Igs ( 20-23
  days )
• 5. starts to be produced at 2-3 month after
  birth and reach the level of adult at 5 -years
  old

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• 6. Functions of IgG
• Important Ig against bacteria and
  virus,neutralize toxin
• Some IgG belong to the auto-antibodies
• eg. long active thyroid stimulator (LATS)
  
• combine with the Fc receptor(Fc?R)

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• combine with the Fc receptor(Fc?R)

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II. IgA 1.Two types serum type monomer
secretary type(sIgA) dimer,trimer or
polymer 2.two subclassesIgA1,IgA2
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• II. IgA
• 1.Two types
• serum type monomer
• secretary type(sIgA)
• dimer,trimer or polymer
• 2.two subclassesIgA1,IgA2

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• 3. to be produced at 4 month after birth
• 4. activate the complement by alternative pathway
  

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5.local immunity of mucosa

• local immunity
• neutralize virus/toxin

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1. MW is highest pentamer(90KD)Valences10
valences in theory
III. IgM
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• 2.half-life is shorter(45 days)
• 3.The earliest synthesized Ig
• be produced during fetus
• appear in the early stage after infection
• 4. The mIg of the B cells act as the antigen
  receptors of B cells(BCR)

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5.Function

• IgM is more effect in
• anti-infection
• anti-bacterium
• natural Ab for blood-type antigen
• auto-antibody rheumatoid factor(RF)

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IV. IgD

• The concentration in serum is low
• and sensitive to proteinase
• 2. Act as the antigen receptor on B cells (mIgD)
  Regulate the differentiation of B cells

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V. IgE

• 1.Concerntration of IgE in serum is the lowest in
  normal individual, but is very high in some
  patients
• 2.related to type I hpersensitivity
• high affinity to Fc?R of mast cells and
  basophils

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• Section III Biological function of Ig

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I. The function of V region -binds to antigen
specifically
Neutralization
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II. The function of C region 1. Activate
complement
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Complement lesion
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II. The function of C region 2. bind to Fc
receptor on some cells
(1)Opsonization(IgG,IgM) enhance
the phagocytosis of MF
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II. The function of C region 2. bind to Fc
receptor on some cells (2)ADCC( antibody
dependent cell mediated cytoxicity
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• II. The function of C region
• 2. bind to Fc receptor on some cells
• (3) Type I hypersensitivity
• -mast cell (Fc?R)
• - basophils (Fc?R)
  

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• II. The function of C region
• 3. Cross placenta or mucosa
• The IgG from mother pass through the placenta
  into fetal body by binding of IgG
• with receptor, FcRn on placenta

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Section IV The Immunogeneity of Ig

• isotype
• allotype
• idiotype

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• 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, subtype

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• The isotypes of Ig
• 1.Classes
• Heavy chains ?,?,?,?,?
• Igs IgG,IgM,IgA,IgD, IgE
• 2.subclasses
• IgG IgG1-4 IgA1-2
• 3. types
• Light chains ?, ?
• Igs IgG ? type or ? type etc.
• 4. Subtypes ? OZ(),OZ(-)

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• 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

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• III. Idiotype of Ig
• Igs produced by one B cell clone possess
  unique structure respectively in hypervariable
  region(HVR) ,this unique structure of Ig is
  called idiotype of Ig

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• In fact
• HVR
• CDR
• idiotype
• are in the same sites of Ig

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Section V. Preparation of Ab

• Polyclonal Ab
• Monoclonal Ab
• Gene engineering Ab

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I.Polyclonal Ab

• a mixture of Abs with different specificities
  and affinities
• generated in a natural response or artificial
  immunization

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II. Monoclonal Ab (mAb)

• Abs produced by single B cell clone (or one
  hybridoma clone ) possess same structure and
  specificity.

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mAb / McAb

• Prepared by hybridoma technique
• Immunized spleen cells (B)
• fuse with myeloma cells and
• form hybridoma with property of proliferating
  and producing antibody

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III. Gene engineering Ab

• Abs prepared by the method of gene recombination

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• Review for chapter 6 immunoglobulin
• Terms
• Antibody , Immunoglobulin,
• CDR , domain of Ig ,Polyclonal Ab,
• Monoclonal Ab, genetic engineering Ab
• Key questions
• .Describe the basic structure of Ig
• .Describe the composition and functions of each
  domain of IgG
• .Describe the functions and hydrolysis fragments
  of Ig digested by Papain /pepsin
• .Please expatiate the features and functions of
  
• five classes Ig
• .How to understand the the biological efforts of
  Ab

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New Thinking way
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To my students Why not the best