2. • Antigen-Antibody (Ag-Ab) Interaction is a biochemical reaction
between antibodies and specific antigens when they come closer to
a distance of several nanometers. It is the binding of paratopes of
antibodies to specific antigens on their epitopes that initiates a
series of immunological responses to act against the respective
antigens for their removal or destruction
Antigen + Antibody Ag-Ab complex → Immune Response
• The antigen-antibody interaction is a bimolecular reversible
association between antigen and antibody. The association
between antigen and antibody includes various non-covalent
interactions between epitope (antigenic determinant) and variable
region (VH/VL) domain of an antibody.
3.
4. Properties of Ag-Ab Reactions
• Highly specific reaction
• – Occurs in an observable manner
• – Non-covalent interaction ( Van der Waals forces, Ionic bonds,
Hydrogen bonds, Hydrophobic interactions )
• – No denaturation of antibodies and antigens
• – Reversible
• – Affinity: It is the strength with which one antigen binds on a
single antigen-binding site on an antibody.
• – Avidity: It is a broader term than affinity. It is a measure of the
overall strength of the Ag-Ab complex. It depends on:
• The affinity of the antibody
• Valency(no. of binding sites) of antibody and antigen
• And the structural arrangement of epitopes and paratopes.
• – Cross-Reactivity: It refers to the ability of an antibody to bind to
similar epitopes of different antigens.
5. Affinity:
• The combined strength of total non-covalent interactions between
a single Ag-binding site of Ab and a single epitope is the affinity of
Ab for that epitope.
• Low-affinity Ab: Bind Ag weakly and dissociates readily.
• High-affinity Ab: Bind Ag tightly and remain bound longer.
Avidity:
• The strength of multiple interactions between multivalent Ab and
Ag is avidity. Avidity is a better measure of the binding capacity of
antibodies than affinity. High avidity can compensate for low
affinity.
7. Precipitation
• Precipitation test is a type of antigen-antibody reaction in
which the antigen occurs in a solution form.
• It is a test in which antibody interacts with the soluble
antigen in the presence of electrolyte to produce a
precipitate.
• Antibody and soluble antigen interacting in aqueous
solution from a lattice that eventually develops into a
visible precipitate.
• Antibody that aggregate the soluble antigens are
called precipitins.
8. Precipitation
•Either side of the equivalence zone, precipitation is actually prevented
because of an excess of either antigen or antibody.
•The zone of antibody excess is known as the prozone phenomenon and
the zone of antigen excess is known as post zone phenomenon.
9.
10. • Slide Test- VDRL test for Syphilis
• Tube test- Syphilis
• Immunodiffusion (Precipitation in Gel)
Single diffusion in one dimension
Double diffusion in one dimension
Single diffusion in two dimensions (Radial)
Double diffusion in two dimensions
Immunoelectrophoresis
11. Agglutination
• The interaction between an antibody and a particle antigen results
in a phenomenon known as agglutination.
• Antibodies that induce such responses are known as agglutinins.
IgM antibody agglutination is superior to IgG antibody
agglutination.
• Antibody excess also reduces the agglutination response; this
inhibition is known as the prozone phenomenon.
• Antibody detection is more sensitive via agglutination than
precipitation.
• Optimal agglutination occurs when antigens and antibodies
react in quantities that are equal.
12. Agglutination
•When an antibody or antigen is overproduced, the prozone phenomenon
may be observed. Although incomplete or monovalent antibodies bind with
the antigen, they do not generate agglutination.
•They may function as blocking antibodies, inhibiting agglutination by the
subsequent addition of the full antibody.
Slide Agglutination- Blood grouping , Widal test fpr Typhoid
Passive agglutination – Rose waaler test- Rheumatoid arthritis autoantibody
13. Complement fixation test
• The fixation of complement occurs during the interaction
of antigen and antibody.
• Thus the consumption of complement in vitro can be used
as a test to detect and measure antibodies, antigens or
both.
• The test depends on the use of a hemolytic indicator
system, consisting of sheep red cells, amboceptor
(antibody to sheep red cells) and complement.
• When added together, sheep red cell antigen complexes
with the antibody utilizes complement and resultant red
cell lysis occurs.
14. • The actual test is done in two stages:
• the first stage consists of adding the test reagents which
are antigen and antibody (normally the biologic fluid
such as human serum) and a standardized amount
complement.
• If antigen and antibody are specific for each other they
will complex and utilize the complement as well.
• The second stage of the test is done by adding the sheep
cell-amboceptor mixture (also known as sensitized sheep
red cells).
• After a period of incubation, the test is read looking
specifically for the presence and degree of red cell lysis.
16. Neutralization
• Neutralization is an antigen–antibody
response in which homologous antibodies
known as neutralizing antibodies negate the
biological effects of viruses and poisons
• These tests fall into two categories:
(a) viral neutralization tests
(b) toxin neutralization testing
