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Immunoglobulin:Ab-Ag Interactions
Lecture 10 20/10/2015
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Antibody structure
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Human Ig classes
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Types of Ig variation
• Isotype• Allotype• Idiotype
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Isotypic variation ( Iso, the same). The genes for isotypic variation
are present in all members of a given species. Examples include
alternative or alternative forms of light chain and the different
heavy chains 1 2 3 4 1 2 and .
Isotypic variation
IgM (kappa)
IgG1 (kappa)
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Allotypic Variation (allos, other) This refers to variation in a
population present in some individuals but not others. This variation
is encoded in alternative alleles of a given gene . Most Ig allotypes
are due to single amino acid differences particularly in the heavy
chain gene sequences.
Allotypic variation
IgG1 (kappa)
Person 2
IgG1 (kappa)
Person 1
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Idiotypic variation (idios, own or personal). Differences in structure of
the antibody variable region or Ag binding site give rise to idiotypic
variation . Hypervariable regions contribute strongly . Abs of the same
class but different specificity will have different idiotypes.
Idiotypic variation
IgG1 (kappa)
Person 1
anti-B
IgG1 (kappa)
Person 1
anti-A
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Summary
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Nature of Ag/Ab Reactions
• Lock and Key Concept
• Non-covalent Bonds
1. Hydrogen bonds2. Electrostatic bonds3. Van der Waal forces4. Hydrophobic bonds
• Reversible
• Multiple Bonds
http://www.med.sc.edu:85/chime2/lyso-abfr.htm
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Ab/Ag interactions
Binding of an antibody to a specific antigen is one key
characteristic of the antibody molecule. How does
this occur?
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In vivo Most antibody responses are polyclonal.
That is we respond by producing a range of
antibodies each with different antigen binding sites.
These antibodies can bind to different antigenic
determinants on or areas of a protein or to several
of the proteins in an complex antigen.
Polyclonal antibody
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Clonal selection hypothesis
+
Stem cell (bone marrow)
Large pool of B cells each with Ab of one specificity IgM or D
Ag Antigen selection
Proliferation and differentiation
Antigen specific B cells Specific antibody
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ANTIGENS, ANTIGENIC DETERMINANTS AND EPITOPES
Antigens
1. Foreign molecules which generate antibodies are called antigens >10kd
2. Pathogens, micro-organisms, proteins
3. Surfaces are antigenic determinants, epitopes, or molecular shapes. Few ~5 Amino Acids in size, or CHO molecule .
Repeated antigenic structure eg bacterial cell wall
Differing antigenic structure eg eukaryotic proteins eg albumin
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Types of Epitopes
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Forces in Ab/Ag Interactions
1. All non-convalent
i) Hydrogen bonding
ii) Electrostatic forces
iii) Van der Waals forces
iv) Hydrophobic bonds
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Forces (continued)2. Complementary Shape and Position.
Critically dependant on distance.
Close and multiple in number.
Requirement for complementary structure:
a) suitable atomic groups must be together
b) shape of site must fit the determinant
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Factors Affecting Measurement of Ag/Ab Reactions
1. Affinity
2. Avidity
3. Ag:Ab ratio
4. Physical form of Ag
Ab excess Ag excess
Equivalence – Lattice formation
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Refers to the interaction of:
1. Monovalent Ag or single determinant.
2. Single Ab combining site
3. Product of the summation of forces.
4. Ab + Ag AbAg
A measure of the binding strength between a single
antigenic determinant and a single Ab combining site
Antibody Affinity
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1. Ab are multivalent (IgM)
2. Multivalent Ag
3. Functional affinity
4. Bonus effect of multivalency
Avidity is not however the sum of the individual binding site affinities.
In practice the divalency of IgG increase its binding to a multimeric Ag by around 104 times.
The pentameric structure of IgM to around 107 times.
Antibody Avidity
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In practice the divalency of IgG increases its binding to a multimeric Ag by around 104 times.
The pentameric structure of IgM to around 107 times.
Affinity Avidity IgGAvidity IgM
104 x greater107 x greater
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Cross Reactivity• The ability of an individual Ab combining site to
react with more than one antigenic determinant.• The ability of a population of Ab molecules to
react with more than one Ag
Anti-A Ab
Ag A
Anti-A Ab
Ag B
Shared epitope
Anti-A Ab
Ag C
Similar epitope
Cross reactions
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The End