Lecture 14 AntibodyAntigen Reactions

1
Lecture 14Antibody-Antigen Reactions
2
Binding of the epitope in the antigen binding site
GOOD FIT
antibody combining site
antigen determinant
3
Antibody Avidity

• Multiple interactions between antigen binding
  sites and epitopes
• Greater than additive
• Similar to Velcro
• More relevant to biologic systems than affinity

4
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
-
-
10
3
-fold
10
7
-fold

definition of bindng
affinity
affinity
avidity
avidity

intrinsic affinity
functional affinity

5
Biological Consequences of Antibody
Affinity/Avidity

• 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

6
Cross-Reaction
A
B
C
D
Immunize
Immunogen
anti-B
anti-C
anti-D
anti-A
Reactivity with rabbit antiserum
A
B
C
D
A
E
F
G
Cross-reactive antigen
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Primary Interactions Between Antigen and Antibody

• 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

8
Enzyme (ELISA) Immunoassay
Enzyme conjugated to anti-Ig antibody (second
antibody)
Add
substrate
for enzyme
anti-antigen
antibody
Antigen
9
Immune Precipitation
Antigen
Antibody
10
Radial Immunodiffusion
Ab-containing gel
Precipitin
ring
Ag/8
Ag/4
Ag/2
Ag/1
D2 Ring
Antigen Concentration
11
Agglutination
IgM Antibody
IgG Antibody
12
Complement activation

• A system of plasma proteins that interact with
• Antigen/antibody complexes
• Pathogen surface motifs (alternative and lectin
  pathways
• Activation of complement results in
• Chemo-attraction of inflammatory cells
• Peptide mediators of inflammation
  (anaphylatoxins)
• Increased blood vessel permeability
• Smooth muscle contraction
• Mast cell degranulation
• Opsonization of pathogens (enhances phagocytosis)
• Killing of pathogens (membrane attack complex)

13
Overview of the Complement Cascade
14
Effector Actions of Complement
15
Terms to remember

• Affinity
• Agglutination
• Avidity
• Chemotaxis
• Complement
• Classical pathway
• Alternative (properdin) pathway

• Cross-reaction
• Epitope/Antigenic Determinant
• Opsonization
• Precipitation
• Second antibody Specificity
• Valence