Antibodies & Antigens
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Transcript of Antibodies & Antigens
Antibodies & Antigens• Pin Ling ( 凌 斌 ), Ph.D.
ext 5632; [email protected]
• 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.
OutlineOutline
• 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
OutlineOutline
• 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?