1. Antibodies & Antigens Pin Ling (凌 斌), Ph.D.
ext 5632;
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

2. Question What effect you expect on the immune system if the thymus was
removed?
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

3. Outline The origin concept of Antibodies
Structures & Features of Antibodies
Antibody binding of Antigens
Applications of Antibodies
Summary & Question

4. Definition of Key Terms
1. Antigen (Ag)
- A molecule can specifically binds to an Antibody or a 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.
2. Antibody (Ab)
- Also called immunoglobulin (Ig), A type of glycoprotein
produced by B cells that binds Ags with high specificity &
affinity.
- Binds to Ags including all classes of molecules, ex. Protein,
Lipid, Carbohydrate, or Chemical.
3. Antiserum
- Serum from an Ag-immunized individual that contains Ab 
specific Ag

5. 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 toxin.

6. Outline Structures & Features of Antibodies
The origin concept of Antibodies
Structures & Features of Antibodies
Antibody binding of Antigens
Applications of Antibodies
Summary & Question

7. 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)

8. Structures of Antibodies-II
Ag-binding site
Antibody (Immunoglobulin, Ig)
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

9. Proteolytic cleavage of Antibody

10. 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)

11. Ab Isotypes-II

12. Ab Isotypes-III

13. Key Concepts in Ab isotypes
1. IgM is the predominant Ab during the primary immune
response and also functions as a 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 (ex. 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

14. Structures of Ab Isotypes
They differ in:
Size
Charge
Amino acid seq
Carbohydrate
content

15. Membrane and secreted forms of Abs
IgD => only membrane form

16. Ig expression during B cell maturation & activation
Membrane-bound IgD or IgM => BCR + Ag
=> B cell activation => Plasma cells => Secreted Abs

17. Changes in Ab structure during humoral immune responses

18. Ab-mediated Immune Effector Systems

19. Secreted IgA dimer => Mucosal lumen

20. Outline Antibody binding of Antigens The origin concept of Antibodies
Structures & Features of Antibodies
Antibody binding of Antigens
Applications of Antibodies
Summary & Question

21. 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

22. Hypervariable Regions form the Ag-binding site
Most of sequence differences among Abs => Three short stretches in V regions
=> Hypervariable
regions (HV),
also called
Complementarity-
determining regions
(CDRs)

23. Complementary interactions between Ag-binding sites and their epitopes

24. Specificity, Cross-reactivity & non-reactivity of AbAg

25. The Nature of Ag determinants

26. Antibody & other Antigen-Recognizing Molecules

27. Valency and Avidity of AbAg interactions
A pentameric IgM
=> low-affinity for each
valent binding
=> Many low-affinity
binding
=> high avidity interaction

28. Flexibility of AbAg interactions

29. Outline Applications of Antibodies The origin concept of Antibodies
Structures & Features of Antibodies
Antibody binding of Antigens
Applications of Antibodies
Summary & Question

30. 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

31. The Development of monoclonal Ab-II

32. Immunodiagnosis-ELISA

33. Immunoprecipitation

34. Outline Summary & Question The origin concept of Antibodies
Structures & Features of Antibodies
Antibody binding of Antigens
Applications of Antibodies
Summary & Question

35. 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.

36. Question
What mechanisms to achieve the
generation of Ab diversity?