Antibodies & Antigens

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 usuallyinduces 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.

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)

Ag-binding site Effector functions Structures of Antibodies-II 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

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

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

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

Immunodiagnosis-ELISA

Immunoprecipitation

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?

Antibodies & Antigens