Transcript Chapter 3 Immunoglobulin

Medical Immunology
Chapter 5 Immunoglobulin
zhang lining
Immunology Institute Medical School
Shandong University
Antibody：a 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
• 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
-globulin
Section I. Molecular Structure of
Immunoglobulin
I. Basic structure of Ig
Four polypeptide chains
2 identical heavy chains
2 identical light chains
N-terminal end
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

C-terminal end
1.Heavy and Light chain:
. Heavy chains (H)
450 ~550aa,
50—75KD
. Light chains (L)
214 aa, 25KD
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
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 
:
20:1 (in mice)
2:1 (in human)
2. variable regions and
constant regions
1) 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
(1) Variable region (V)
Hypervariable region (HVR)
Framework region (FR)
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 threedimensional structure of the bound antigen, HVR
are also called Complementarity –determining
regions, CDRs
L chain possesses 3 CDRs：CDR1, CDR2 and CDR3
H Chain possesses 3 CDRs : CDR1, CDR2 and CDR3
• 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
L:
CDR1—28-35,
CDR2—49-56,
CDR3—91-98
H: CDR1-29-31
CDR2-49-58
CDR3-95-102
CDR1, CDR2,CDR3
CDR1
CDR2
CDR3
Ag-binding sites
L: CDR1—28-35,
CDR2—49-56,
CDR3—91-98
H:
CDR1-29-31
CDR2-49-58
CDR3-95-102
The antigen-binding site
binds to epitope of
antigen
CDR3
CDR1
CDR2
Ag-binding sites
3. Hinge region
• The hinge region is segment of heavy chain
between the CH1 and CH2 domains.
• Flexibility in this area permits the two antigenbinding sites to operate independently.
II. Other components of Ig
• Joining chain(J )

Secretory piece( SP)
Joining chain (J )

produced by plasma cells

Functions: linker
Join monomer of Ig to form dimer, or polymer
( Ig A, IgM)
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.
Secretory piece( SP)
J chain
Secreted
piece
Formation of secreted piece
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
• 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
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.
IV. Proteolytic fragments of an IgG
molecules and their functions
Ig digested by papain and pepsin
.position
.Fragments:
.Function:
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
•
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
Significance
· Elucidating the relationships between the
structure and function of Igs .
 Decrease the immunogenicity of Ig for
clinical treatment
Section II .The features and
functions of different classes Ig
I . IgG
1. Highest concentration in
serum (75% of total Ig)
2. Four subclasses :
IgG1, IgG2, IgG3, IgG4
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
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(FcγR)
•combine with the Fc receptor(FcγR)
II. IgA
1.Two types
serum type ：monomer
secretary type（sIgA）：
dimer，trimer or polymer
2.two subclasses：IgA1，IgA2
II. IgA
1.Two types
serum type ：monomer
secretary type（sIgA）：
dimer，trimer or polymer
2.two subclasses：IgA1，IgA2
3. to be produced at 4 month after birth
4. activate the complement by alternative
pathway
5.local immunity of mucosa
• local immunity
neutralize virus/toxin
III. IgM
1. MW is highest ：pentamer（90KD）
Valences：10 valences in theory
2.half-life is shorter(4~5 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)
5.Function:
• IgM is more effect in
anti-infection
anti-bacterium
• natural Ab for blood-type antigen
• auto-antibody :rheumatoid factor(RF)
IV. IgD
1. The concentration in serum is low
and sensitive to proteinase
2. Act as the antigen receptor on B cells
(mIgD):
B cells
Regulate the differentiation of
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
• Section III Biological function of Ig
I. The function of V region -binds to antigen specifically
Neutralization
II. The function of C region
1. Activate complement
Complement lesion
II. The function of C region
2. bind to Fc receptor on some cells
(1)Opsonization(IgG,IgM)
enhance the phagocytosis of MΦ
II. The function of C region
2. bind to Fc receptor on some cells
（2)ADCC( antibody dependent cell mediated cytoxicity
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)
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
Section IV
The Immunogeneity of Ig
• 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, subtype
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（-）
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
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
In fact:
HVR
CDR
idiotype
are in the same sites of Ig
Section V. Preparation of Ab

Polyclonal Ab

Monoclonal Ab

Gene engineering Ab
I.Polyclonal Ab

a mixture of Abs with different
specificities and affinities
 generated in a natural response
or artificial immunization
II. Monoclonal Ab (mAb)
Abs produced by single B cell clone
(or one hybridoma clone ) possess
same structure and specificity.
mAb / McAb
 Prepared
by hybridoma technique:
Immunized spleen cells (B)
fuse with myeloma cells and
form hybridoma with property of
proliferating and producing
antibody
III. Gene engineering Ab
Abs prepared by the method of
gene recombination
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
New Thinking way
To my students:
Why not the best