LECTURE 3
ANTIGEN-ANTIBODY INTERACTIONS AND THEIR
APPLICATIONS IN LABORATORY INVESTIGATIONS
STRUCTURE OF ANTIBODY
Basic structure of Immunoglobulin – Y-shape
Four polypeptide chains – 2 light chain and 2 heavy chains which
are linked to form a Y-shape.
Constant region determines the Ab class.
Variable regions determine antigen binding .
Variable regions in both heavy and light chains consist of 3
Hypervariable regions (HV1, HV2, HV3) and 4 framework regions
(FR1 – 4).
HV regions are in direct contact with the antigens/epitopes and
therefore determine complementarity and antigen binding. Hence
HV regions are also called complementarity determining regions.
56
,Nature of Ag/Ab Reactions
• Lock and Key Concept
• Non-covalent Bonds
– Hydrogen bonds
– Electrostatic bonds
– Van der Waal forces
– Hydrophobic bonds
• Reversible
• Multiple Bonds
AFFINITY
•Strength of the reaction between a single antigenic determinant
and a single Ab combining site.
Affinity = attractive and repulsive forces.
57
, AVIDITY
•The overall strength of binding between an Ag with many
determinants and multivalent Abs.
SPECIFICITY AND SENSITIVITY
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.
Sensitivity:
This term often applies to tests
It is the ability of a test to detect minimal levels of antibody of
antigen in a sample
Some tests are more sensitive than others i.e. some tests like
ELISA is more sensitive than precipitation. It can detect upto
nanograms of antibody in a sample, whereas a precipitation test
would detect 200ug/ml antibody in a sample and not below this.
CROSS REACTIVITY
58
ANTIGEN-ANTIBODY INTERACTIONS AND THEIR
APPLICATIONS IN LABORATORY INVESTIGATIONS
STRUCTURE OF ANTIBODY
Basic structure of Immunoglobulin – Y-shape
Four polypeptide chains – 2 light chain and 2 heavy chains which
are linked to form a Y-shape.
Constant region determines the Ab class.
Variable regions determine antigen binding .
Variable regions in both heavy and light chains consist of 3
Hypervariable regions (HV1, HV2, HV3) and 4 framework regions
(FR1 – 4).
HV regions are in direct contact with the antigens/epitopes and
therefore determine complementarity and antigen binding. Hence
HV regions are also called complementarity determining regions.
56
,Nature of Ag/Ab Reactions
• Lock and Key Concept
• Non-covalent Bonds
– Hydrogen bonds
– Electrostatic bonds
– Van der Waal forces
– Hydrophobic bonds
• Reversible
• Multiple Bonds
AFFINITY
•Strength of the reaction between a single antigenic determinant
and a single Ab combining site.
Affinity = attractive and repulsive forces.
57
, AVIDITY
•The overall strength of binding between an Ag with many
determinants and multivalent Abs.
SPECIFICITY AND SENSITIVITY
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.
Sensitivity:
This term often applies to tests
It is the ability of a test to detect minimal levels of antibody of
antigen in a sample
Some tests are more sensitive than others i.e. some tests like
ELISA is more sensitive than precipitation. It can detect upto
nanograms of antibody in a sample, whereas a precipitation test
would detect 200ug/ml antibody in a sample and not below this.
CROSS REACTIVITY
58
