Antibodies & Antigens
• Pin Ling (凌 斌), Ph.D.
ext 5632; lingpin@mail.ncku.edu.tw
• References:
1. Abbas, A, K. et.al, Cellular and Molecular
Immunology (6th ed., 2007), Chapter 4
2. Male D., J. Brostoff, D. B Roth, and I. Roitt
Immunology (7th ed., 2006), Chapter 3
Question
What effect would you expect the
thymus removal (thymectomy) to have
on the ability of host immunity against
infection?
Ans: 1. Total lymphocytes are drastically reduced. T cell
development was blocked. B cells are also reduced
=> require T helper cells for their proliferation.
LN size is reduced. => Get infections easier.
2. DiGeorge Syndrome => patients w/ congenital
thymic aplasia => Fewer T cells in defected thymus
Outline
• The origin concept of Antibodies
• Structures & Features of Antibodies
• Antibody binding of Antigens
• Applications of Antibodies
• Summary & Question
Definition of Key Terms
1. Antibody (Ab)
- Also called immunoglobulin (Ig), a type of glycoprotein
produced by B cells that binds Antigen (Ag) with high specificity &
affinity.
- Binds to Ag including all classes of molecules, eg. Protein,
Lipid, Carbohydrate, or Chemical.
2. Antigen (Ag)
- A molecule can specifically binds to Antibody (Ab) or T-cell
receptor (TCR) and usually induces an adaptive immune response.
- Antigen Determinant (Epitope): The specific portion of an Ag
recognized by an Ab or TCR.
3. Antiserum
- Serum from an Ag-immunized individual that contains Ab 
specific Ag
In 1890, Behring & Kitasato => Serum therapy
Inactivated toxin => Animal-A => Protective immunity
Serum from Animal-A => Animal-B => Passive immunity
Serum A contains “Anti-toxin” proteins => Humoral
immunity
“Anti-toxin” => “Anti-body” that recognize many
other substances in addition to toxin.
Outline
• The origin concept of Antibodies
• Structures & Features of Antibodies
• Antibody binding of Antigens
• Applications of Antibodies
• Summary & Question
Structures of Antibodies-I
Antibody (Immunoglobulin,
Ig)
1. Two identical Light chains
Two “ “
Heavy chains
2. Each chain has repeating unit,
Ig domain
3. Chains are linked by disulfide
bonds
4. Each chain consists of
- Variable region (N-terminus)
- Constant region (C-terminus)
Structures of Antibodies-II
Antibody (Immunoglobulin,
Ig)
Ag-binding site
5. Share basic structure
features
6. Show the remarkable
variability in regions for Ag
bindings
=> H-Variable region
+ L-Variable region
=>Antigen binding site
7. Heavy chain Constant region
=> Effector functions
Effector functions
Proteolytic
cleavage
of Antibody
Ab Isotypes-I
1. 5 distinct classes:
Ig A, D, E, G, & M
Some => subclasses
Heavy chain
=> Classification
2. Different Ig isotypes
=> Different Effector
functions
3. IgG => predominant &
long half-life
IgA => Mucosal
lumens & milk
4. Membrane-bound
Ig M & D => B-Cell
antigen Receptor (BCR)
Ab Isotypes-II
Ab Isotypes-III
Key Concepts in Ab isotypes
1. IgM is the predominant Ab during the primary immune
response and also functions as a B-Cell Receptor (BCR).
2. IgG is the predominant Ab during the secondary immune
response.
3. IgA is produced in 2nd immune response and plays a key role
in mucosa immunity area (eg. Respiratory & GI tracts).
4. IgD is a membrane-bound Ag receptor on B cells.
5. IgE have evolved to protect against helminth parasites.
6. Fc receptors expressed on various immune cells
=> Mediate Ab effector functions
Structures of
Ab Isotypes
They differ in:
• Size
• Charge
• Amino acid seq
• Carbohydrate
content
Membrane and Secreted forms of Abs
IgD => only membrane form
Ig expression during B cell
maturation & activation
Membrane-bound IgD or IgM => BCR + Ag
=> B cell activation => Plasma cells => Secreted Abs
Changes in Ab structure during
humoral immune responses
Ab-mediated Immune Effector Systems
Secreted IgA dimer => Mucosal lumen
Outline
• The origin concept of Antibodies
• Structures & Features of Antibodies
• Antibody binding of Antigens
• Applications of Antibodies
• Summary & Question
Key Concepts in Ab-Ag interaction
1. Antibody (Ab) form multiple non-covalent bonds with
antigen=> Reversible
- Attractive forces (H bonds, electrostatic bonds, van der
Waals forces, & hydrophobic forces) => High affinity
interaction
2. The Ag-binding sites of an Ab are complementary to the
conformation of Ag determinants (epitopes) of an Ag.
3. Affinity vs. Avidity for AbAg
Affinity => A measure of the strength of interaction between
an Ag-binding site and its epitope
=> Kd, dissociation constant; small Kd => stronger
affinity
Avidity => The overall strength of AbAg
=> Affinity & the valency of interactions
Hypervariable
Regions form
the Ag-binding
site
Most of sequence
differences among Abs
=> Three short
stretches in V regions
=> Hypervariable
regions (HV),
also called
Complementaritydetermining regions
(CDRs)
Complementary interactions between
Ag-binding sites and their epitopes
Specificity, Cross-reactivity &
non-reactivity of AbAg
The Nature of Ag determinants
Antibody & other AntigenRecognizing Molecules
Valency and Avidity of AbAg
interactions
A pentameric IgM
=> low-affinity for each
valent binding
=> Many low-affinity
binding
=> high avidity interaction
Flexibility of AbAg interactions
Outline
• The origin concept of Antibodies
• Structures & Features of Antibodies
• Antibody binding of Antigens
• Applications of Antibodies
• Summary & Question
The Development
of monoclonal Ab
Unlimited production of
unique Ab for a specific Ag
=> Revolutionize Immunology
& other fields
Its Applications:
- Identification of
phenotypic markers
- Immunodiagnosis &
Immunotherpy
-Tumor diagnosis & therapy
The Development of
monoclonal Ab-II
ImmunodiagnosisELISA
Immunoprecipitation
Outline
• The origin concept of Antibodies
• Structures & Features of Antibodies
• Antibody binding of Antigens
• Applications of Antibodies
• Summary & Question
SUMMARY
1. All Abs have s common symmetric structure:
(2 Heavy chains + 2 Light chains ) meanwhile show the
remarkable variability in regions for Ag bindings
2. Abs are classified into different isotypes on the basis of
different Heavy chain C regions.
Ab-mediated effector functions also depend on the H-chain
C regions.
3. Five classes of Ab in mammals:
IgA, IgD, IgE, IgG & IgM
4. Monoclonal Abs are applied to many fields for research &
clinical treatment.
Question
What mechanisms to achieve the
generation of Ab diversity?