Transcript Chapter 3 Immunoglobulin

Chapter 5
Immunoglobulin
Contents
Introduction
SectionⅠ Molecular Structure of Ig
SectionⅡ Characteristics and Functions of the 5
Classes of Ig
Section Ⅲ Fc Receptors for Ab Molecules
Section Ⅳ Biological Activity of Ab
SectionⅤ Immunogenicity of Ig
Section Ⅵ Artificial Ab
Concepts
Antibody (Ab):
Glycoprotein molecules
that are produced by plasma cells and can
combine with the corresponding Ag
specifically are called Ab.
 Ab is produced by B cells in the response to
a stimulation of Ag.
 Ab possesses a high degree of specificity
and affinity
• Immunoglobulin (Ig):
It refers to all globulins that possess the
activity of Ab or show a similar structure to
Ab
• Therefore, All Abs are Igs, but not all Igs
possess the functions of Abs
Other Concepts
γ- Globulin
Antiserum
Humoral Immunity
sIg and mIg(BCR)
SectionⅠ Molecular Structure of
Ig
Ⅰ. Basic structure

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 shows “T” or “Y” shape.
1. H and L chain:
. Heavy chains (H):
450 ~ 550 aa,
50 ~ 75 KD
. Light chains (L):
214 aa, 25 KD
Two terminal ends for each peptide
chain
“N”
“N”
“C”
terminal end
terminal end
“C”
L chains attach to H chains
from “N” end
2. classes and types of Ig
(1) According to the differences of H chains
(amino acid composition, sequence)
Igs can be divided into 5 classes
subclasses
•
•
Five classes of H Chain:     
Five classes of Igs: IgG IgA IgM IgD IgE
IgG1~ IgG4
IgA1, IgA2
(2) According to the differences of L chains:
Two types of L chain: , 
 1~  4
 : 
20:1 (in mice); 2:1 (in human)
3. Two regions of each peptide chain
(1) Constant region (C)
(2) Variable region (V)
(3) Hinge region
3. Two regions of each peptide chain
(1) Constant region ( C )
•
CH:
3/4 or 4/5 (,) of H chain from the c end
•
CL:
1/2 of L chain from the c end
(2) Variable region ( V )
•
CH:
1/4 or 1/5 (,) of H chain from the N end
•
CL:
1/2 of L chain from the N end
(2) Variable region ( V ):
 Hypervariable region(HVR)
There are three highly
diversity stretches within the
V egion,
they are called HVR.

Framework region(FR): FR1-FR4
Ag-binding sites
Complementarity determining regions(CDR)
(2) Variable region (V)
Complementarity determining regions(CDR)
L: CDR1, CDR2, CDR3
H: CDR1, CDR2, CDR3
Idiotype of Ig
Igs produced by different B cells possess unique
structure respectively in hypervariable region
(HVR), the unique structure of Ig is called
idiotype or idiotypic determinant
In fact:
HVR
CDR
Idiotype
are in the same sites of Ig
(3) Hinge region:
• Flexible and suitable for CDR of Ig
binding to antigenic determinants.
• Sensitive to proteolytic enzyme
• IgM, IgE
Other structures of Ig
• Joining chain(J)
 Secretory piece(SP)
Joining chain(J ) :

Produced by plasma cells

Functions:linker, to compose dimer、
pentamer or polymer(IgA,
IgM)
Secretory piece( SP):
. Produced by mucosa epithelial cells
. Secretory IgA (sIgA)
. Functions: protect sIgA, resist
proteolysis in extra secretory liquid.
IgA
Ⅱ. Domains of Ig
1. Domain :
Polypeptide chains of Ig are folded into a
globular structure by intra chain s-s bond
within each 110aa region which is called a
domain
2. Domains of Ig
• L chain(2) : VL, CL
• H chain(4~5):
VH,
CH1, CH2, CH3
CH4(in IgM,IgE)
hinge region
3. Function of each
domain
• VH, VL: antigen-binding site
• CH1, CL: allogeneic marker
• CH2/CH3: complement-fixing site,
permeate placenta(IgG)
• CH3/CH4: cell-binding site
 Hinge region :flexible and suitable for CDR of
Ig binding to antigenic determinants
Ⅲ. Hydrolytic fragments of Ig
Ig can be digested by papain and
pepsin
• Position
• Fragments
• Function
1. Digested by papain
• Position:
near the S-S bonds of H inter-chains fromthe N end

Fragments:

Function:
2Fab:fragment antigen-binding
Fc:fragment crystallizable
Fab: recognize and bind Ag
Fc:
(1) fix complement
(2) crossing the placenta
(3) bind to FcR in different cells
•
2. Digested by pepsin
• Position:
near the S-S bond of H inter-chains from the C end
• Fragments and function :
F(ab′)2: bind antigen(2 valence)
pFc′: no function
Significance
 Elucidating the relationships
between the structure and function
of Igs
 Decrease the immunogenicity of
Ig for clinical treatment
SectionⅡ Characteristics and
Functions of the 5 Classes of
Igs
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 longer( 16-24 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:
•
•
•
•
•
Against bacteria and virus,neutralize toxin
Combine with the Fc receptor(FcγR)
Activate complement
Combine with SPA
Some belong to the auto-antibodies
• Take part in type Ⅱ and Ⅲ
hypersensitivity
Ⅱ. IgM
1. Highest MW：pentamer（90 KD）,10 valences
2. Half-life is shorter(4~5 days)
3. The first Ig to be synthesized
•
•
•
Appear in the early stage after infection
Be produced during fetus
The first mIg of the B cells, act as the antigen
receptors(BCR)
4. Functions:
• IgM is more effective in binding Ag and
activating C, and play an important role
in anti-infection
• Natural Ab for blood-type antigen
• Auto-antibody: rheumatoid factor(RF)
• Take part in type Ⅱ and Ⅲ
hypersensitivity
Ⅲ. 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 months after
birth
4. Exist in almost all body fluid
6. Local mucosal immunity
•
•
•
•
Immune barrier
Neutralize virus/toxin
Rich in colostrum
Activate C by alternative pathway
•
Take part in type Ⅲ hypersensitivity
Ⅳ. IgD
1. 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
Ⅴ. IgE
1.Concerntration of IgE in serum is the
lowest in normal individual, but is very
high in some patients.
2.Related to typeⅠpersensitivity
FcεRⅠ: mast cell, basophil
Section Ⅲ Fc Receptors for
Ab Molecules
IgG---FcR: FcRⅠ(CD64)---phagocyte
FcRⅡ(CD32)---immune complex
FcRⅢ(CD16)---NK, MΦ,T cell
IgE---FcR: FcRⅠ--- mast cell, basophil
FcRⅡ--- macrophage, B cell
IgA---FcαR(CD89)---phagocyte, neutrophil
SectionⅣ Biological Activity of Ab
1. Recognize and bind to antigen
specifically
2. Fix complement
3. Bind to Fc receptor on some cells
4. Transfer selectively :
.Planceta transfer (IgG)
.Mucosa transfer (sIgA)
Affinity and Avidity
Neutralization
IgM,IgG1~3:
classical pathway
IgA,IgG4,IgE:
alternative pathway
MAC
(1) Opsonization(IgG, IgM):
Enhance the phagocytosis of MΦ
(2) ADCC( antibody dependent cell
mediated cytotoxicity)
(3) Hypersensitivity typeⅠ
- mast cell, basophil(FcRⅠ)
allergen
IgE
FcRI
degranulation
inflammation
SectionⅤ Immunogenicity of Ig
Isotype: CH, CL
Allotype：CH, CL
Idiotype: VH, VL
Anti-idiotype antibody
SectionⅥ Artificial Ab



Polyclonal Ab
Monoclonal Ab
Gene engineering Ab
1. Polyclonal Ab
 A mixture Ab with different
specificities and affinities
 Generate in a natural response
or artificial immunization
 Cross reaction
Cross-reactivity:
if two antigens share an epitope
an antibody recognizes an unrelated,
but chemically similar, epitope
2. Monoclonal Ab (mAb)
Ab produced by single clone (or one
hybridomas clone ) and having a single
specificity
mAb / McAb
 Prepared
by hybridomas technique:
Immunized spleen cells(B) hybride with
myeloma cells----hybridomas
Artificial antibodies
POLYCLONAL.
MONOCLONAL.
Derived from different B
Lymphocytes cell lines
Derived from a single B cell
clone
Batch to Batch variation
affecting Ab reactivity &
titre
mAb offer Reproducible,
Predictable & Potentially
inexhaustible supply of Ab
with exquisite specificity
NOT Powerful tools for
clinical diagnostic tests
Enable the development of
secure immunoassay systems.
3.Gene engineering Ab
• Abs prepared by the method of gene
recombination
• Chimeric Ab:human Fc bind with mice Fab
• Recombinant single chain Ab:VH-linker-VL
Human-mouse chimeric Ab