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

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

Outline The origin concept of AntibodiesThe 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 AntibodiesStructures & Features of Antibodies Antibody binding of Antigens Applications of Antibodies Summary & Question

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-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-II Ag-binding site 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 Complementarity- determining 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 Antigen- Recognizing 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

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

Outline The origin concept of Antibodies Structures & Features of Antibodies Antibody binding of Antigens Applications of Antibodies Summary & QuestionSummary & 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?