3 th Immunity
practical
class
Lab 6
Hani Hassan
Ag-Ab reactions
The reaction between antigens and antibodies involves complementary
binding sites on the antibody and on the antigen molecules. The sites on
the antigen molecule that combine with the binding site of an antibody
are known as epitopes. In the same way that the binding site is
determined by different segments on the variable regions of heavy and
light chains that come in close proximity due to the folding of those
regions, the epitopes are also formed by discontinuous segments of an
antigen molecule.
The interaction between the antibody-binding site and the antigen epitope
involves exclusively non covalent bonds such as:
1- Electrostatic Bonds.
2- Hydrogen Bonding.
3- Hydrophobic Interactions.
4- Van der Waals Bonds.
All these types of interactions depend on the close proximity of the
antigen and antibody molecules. For that reason, the “good fit”
between an antigenic determinant and an antibody combining site
determines the stability of the antigen-antibody reaction.
Specificity:
● The ability of an individual antibody combining site to react with
only one antigenic determinant.
● The ability of a population of antibody molecules to react with only
one antigen.
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3 th Immunity
practical
class
Lab 6
Hani Hassan
Affinity:
Antibody affinity can be defined as Strength of the reaction between a
single antigenic determinant and a single Ab combining site.
Affinity = { attractive and repulsive forces}
Avidity:
Antibody avidity can be defined as the overall strength of binding
between an Ag with many determinants and multivalent Abs.
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3 th Immunity
practical
class
Lab 6
Hani Hassan
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.
Factors Affecting Measurement of Ag/Ab Reactions:
1- Affinity.
2- Avidity.
3- Ag:Ab ration.
4- Physical form of Ag.
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3 th Immunity
practical
class
Lab 6
Hani Hassan
Tests Based on Ag/Ab Reactions:
● All tests based on Ag/Ab reactions will have to depend on lattice
formation or they will have to utilize ways to detect small immune
complexes.
●All tests based on Ag/Ab reactions can be used to detect either Ag or
Ab.
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