Transcript Good fit and poor fit

Lecture 14
Antibody-Antigen Reactions
Binding of the epitope in the
antigen binding site
GOOD FIT
POOR FIT
antibody combining site
antigen determinant
high attraction
low repulsion
high repulsion
low attraction
Antibody Avidity
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Multiple interactions between antigen binding
sites and epitopes
Greater than additive
Similar to Velcro™
More relevant to biologic systems than
affinity
Effect of multivalent
interactions
antibody
Fab
IgG
IgG
IgM
effective antibody valence
1
1
2
up to 10
antigen valence
1
1
n
n
equilibrium constant (L/M)
10 4
10 4
10 7
10 11
advantage of multi-valence
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-
10 3 -fold
10 7 -fold
affinity
affinity
avidity
avidity
definition of bindng
intrinsic affinity
functional affinity
Biological Consequences of Antibody
Affinity/Avidity
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Neutralization of toxins
Complement activation
Immune elimination of antigen
Virus neutralization
More intense immune complex disease in animals
 higher levels of circulating antigen-antibody complexes
 more intense localization of immune complexes on basement
membranes.
 more severe impairment of organ function
Cross-Reaction
A
B C
D
Immunize
Immunogen
anti-A
A
B C
D
A
E F G
Cross-reactive antigen
anti-B
anti-C
anti-D
Reactivity with
rabbit
antiserum
Primary Interactions Between
Antigen and Antibody
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Determined by the interaction between
epitopes and antigen-binding sites
Independent of biochemical and biologic
phenomena that may follow
Depend only on the quantity and quality
(affinity/avidity) of antibodies
The basis for ELISA, RIA, IFA assays
Enzyme (ELISA) Immunoassay
Enzyme conjugated
to anti-Ig antibody
(“second antibody”)
anti-antigen
antibody
Antigen
Add
substrate
for enzyme
Immune Precipitation
Antigen
Antibody
Radial
Immunodiffusion
Ab-containing gel
Precipitin
ring
Ag/8 Ag/4
Ag/2
Ag/1
D2 Ring
Antigen Concentration
Agglutination
RBC
IgM Antibody
RBC
RBC
RBC
RBC
RBC
RBC
RBC
IgG Antibody
RBC
RBC
RBC
Complement activation
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A system of plasma proteins that interact with
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Antigen/antibody complexes
Pathogen surface motifs (alternative and lectin pathways
Activation of complement results in
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Chemo-attraction of inflammatory cells
Peptide mediators of inflammation (anaphylatoxins)
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Increased blood vessel permeability
Smooth muscle contraction
Mast cell degranulation
Opsonization of pathogens (enhances phagocytosis)
Killing of pathogens (membrane attack complex)
Overview of the Complement Cascade
Effector
Actions of
Complement
Terms to remember
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Affinity
Agglutination
Avidity
Chemotaxis
Complement
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Classical pathway
Alternative
(properdin) pathway
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Cross-reaction
Epitope/Antigenic
Determinant
Opsonization
Precipitation
Second antibody
Specificity
Valence