Immunoglobulin molecules have a variable region made up of light and heavy chains that provides specificity for antigens. The variable region, called the Fab segment, contains hypervariable complementarity determining regions (CDRs) that form antigen contact points and allow for recognition of a diverse range of antigens. The Fc region mediates effector functions like complement binding and interaction with immune cells. The structural basis for antibody diversity lies in the highly variable amino acid sequences of the CDR loops within the Fab segment.

1. Immunoglobulins

2. Structure of Immunoglobulin molecule
Variable Region (VL & VH)
Disulphide Bonds
Light chain(kappa and
lambda)
Heavy Chain
Hinge

3. Structure of Immunoglobulin molecule
•
Fab
– Ag binding
– Specificity determined by VH
and VL
– Block the active sites of toxins
or pathogen-associated
molecules.
Papain
•
Fc
– Effector functions
– Trafficking of antigens into the
antigen processing pathways.
– Complement Binding
– Binding to the cells like mast
cells, granulocytes,
macrophages

4. In an immunoglobulin molecule, what is the structural
basis for the infinite diversity needed to match the
antigenic universe?

5. Variable Region of Fab Segment
COMPLEMENTARITY
DETERMINING
REGIONS (CDR) –
These are hypervariable
regions that have
antigen contact points.
FRAMEWORK
REGION

6. Hypervariable CDRs are located on loops at the end.
(Yellow Color)

7. Hypervariable loops and framework: Summary
•The framework supports the hypervariable loops.
•The sequences of the hypervariable loops are highly variable amongst
antibodies of different specificities.
•The variable sequences of the hypervariable loops is also influenced by the
shape, hydrophobicity and charge at the tip of the antibody.
•Variable amino acid sequence in the hypervariable loops accounts for the
diversity of antigens that can be recognised by a repertoire of antibodies.

8. Isotypes, Allotypes, and Idiotypes of Immunoglobulins
1. ISOTYPES
All the heavy chain constant region structures which give rise to classes
and subclasses are expressed together in the serum of a normal subject
are called Isotypic variants. (IgG, IgA, IgM, etc)
Each of those is called Isotype.
2. ALLOTYPES
Inherited small, functionally insignificant, structural differences, especially
in the constant portion of the antibody cause by the difference of amino acid
sequence.
The variations in heavy chain genes are called Allotypes.
3. IDIOTYPE
The variations of amino acid sequences within the variable domains on light
and heavy chains are called Idiotypes.

10. …to make things easy!
Isotype: Present in ALL members of the same species (ALL humans); ie., same
for all humans.
Allotype: Present in members of the same family, same ethnic group, etc. Used
for paternity testing.
Idiotype: Different for each and every individual.

11. Thank you very much!