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

QuestionQuestion

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 thymi

c aplasia => Fewer T cells in defected thymus

What effect would you expect the thymus removal (thymectomy) to have on the ability of host immunity against infection?

OutlineOutline

• The origin concept of Antibodies The origin concept of Antibodies

• Structures & Features of Antibodies

• Antibody binding of Antigens

• Applications of Antibodies

• Summary & Question

Definition of Key Terms1. 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 recogn

ized by an Ab or TCR. 3. Antiserum - Serum from an Ag-immunized individual that contains Ab specific Ag

In 1890, Behring & Kitasato => Serum therapyInactivated toxin => Animal-A => Protective immunity Serum from Animal-A => Animal-B => Passive immunitySerum A contains “Anti-toxin” proteins => Humoral immunity

“Anti-toxin” => “Anti-body” that recognize many other substances in addition to toxin.

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• The origin concept of Antibodies

• Structures & Features of AntibodiesStructures & Features of Antibodies

• Antibody binding of Antigens

• Applications of Antibodies

• Summary & Question

Antibody (Immunoglobulin, Ig)1. Two identical Light chains Two “ “ Heavy chains2. 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-I

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

Structures of Antibodies-IIAg-binding site

Effector functions

Proteolytic cleavage of Antibody

Ab Isotypes-I1. 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 isotypes1. 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

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• 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 interactio

n 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 AbAg 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 AbAg => Affinity & the valency of interactions

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

Complementary interactions between Ag-binding sites and their epitopes

Specificity, Cross-reactivity & non-reactivity of AbAg

The Nature of Ag determinants

Antibody & other Antigen-Recognizing Molecules

Valency and Avidity of AbAg interactions

A pentameric IgM=> low-affinity for each valent binding=> Many low-affinity binding => high avidity interaction

Flexibility of AbAg interactions

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• The origin concept of Antibodies

• Structures & Features of Antibodies

• Antibody binding of Antigens

• Applications of Antibodies

• Summary & Question

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

The Development of monoclonal Ab-II

Immunodiagnosis-ELISA

Immunoprecipitation

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• The origin concept of Antibodies

• Structures & Features of Antibodies

• Antibody binding of Antigens

• Applications of Antibodies

• Summary &Summary & Question Question

SUMMARY1. All Abs have s common symmetric structure: (2 Heavy chains + 2 Light chains ) meanwhile show the remarka

ble 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.

QuestionQuestion

What mechanisms to achieve the generation of Ab diversity?