Antibody Diversity and Antibody Class Switching Flashcards
What is antibody diversity? (1)
Refers to the array of different antigenic specificities produced in the humoral immune response
Antigenic specificity of an antibody is dictated by what? (1)
Antigenic specificity of an antibody is dictated by the sequence of the VL and VH domains
How does antigenic specificity arise? (2)
Generated by recombination of the antibody genes during the antigen-independent differentiation of B-cells in the bone marrow
How much antibody diversity is produced? (3)
~5 X 1013 different specificities of antibodies can be produced, enabling the immune response to make an antibody for every antigen it encounters
Each B-cell/ plasma cell clone only makes one antibody specificity
The total number of protein encoding genes in the human genome (2 – 2.5 X 104) insufficient to accommodate one complete gene for each antibody molecule
How many antibody genes are there? (2)
We have 3 antibody genes 2 light chain (kappa and lambda) and 1 heavy chain gene
How can we make antibodies with ~5 X 1013 different specificities when there are only 3 antibody genes? (1)
Diversity is achieved by recombination of less than 1,000 gene segments in the DNA of developing B-cells
When is diversity of antibody genes generated? (1)
During the antigen independent development of B-cells
How does recombination of gene segments cause antibody specificity for antigen? (3)
The VH and VL domains form the antigen binding site
The sequence of the VH and VL domains is dictated by the gene segments used to encode these protein domains
The sequence of the VH and VL domains dictates the antigenic specificity of the antibody
How is the coding region of the variable domain of the light chain generated? (2)
Each light chain uses one V, and one J gene segment to generate the coding region for the variable domain
How is the coding region of the variable domain of the heavy chain generated? (2)
Each heavy chain uses one V, one D and one J gene segment to generate the coding region for the variable domain
Which process generates the coding region for the variable domain? (1)
V (D) J recombination
Where are heavy chains associated with light chain in a cell? (1)
Heavy chains associate with light chains in the ER to make antibody molecules
What is combinatorial diversity? (2)
Combinatorial diversity refers to the different combinations of heavy and light chains that can be used to form an antibody
Combinatorial diversity further increases the number of available antigen specificities
How does antigen-independent B cell differentiation occur in the bone marrow? (4)
Lymphoid progenitor»_space;> Recombination of
immunoglobulin genes»_space;> Expression of antibody surface receptors (IgD and IgM)»_space;> B-cells then migrate to secondary lymphoid tissues
Lifetime of B cell
B-cells migrate to secondary lymphoid tissues but will die if they do not recognise specific antigen within ~ 21 days
What does class switching do? (3)
Changes the constant domain, but does not change the variable domain and hence antigenic specificity is not changed
Tailors the humoral response to the antigen
Immunoglobulin genes (2)
Immunoglobulin genes are non-functional until
they are rearranged
Immunoglobulin genes are rearranged during
antigen independent differentiation of B-cells
in the bone marrow
Describe the basic principle of light chain gene recombination (7)
recombination of V and J segments
Chop out DNA in between
Remainder segment
transcribed to produce mRNA
Spliced to remove joining segments not used in the sequence
light chain mRNA is translated into the immunoglobulin light chain polypeptide in
the endoplasmic reticulum (ER)
Signal sequence targets protein to the ER
The light chains then associate with the heavy chains in the ER
to make the antibody molecule
Describe the basic principle of heavy chain gene recombination (7)
Recombination of D and J segement
Recombination of V and D segment
VDJ segment joined together and transcribed
Spliced
Produce mRNA in virgin B cell – VDJ combined to constant domain
mRNA in a virgin B-cell is translated into antibody heavy chain proteins in the endoplasmic reticulum (ER)
Heavy chains associate with light chains in the ER to make antibody molecules
Arithmetic of VDJ recombination
CHECK LECTURE SLIDE
Describe junctional diversity (4)
Nucleotides are added and removed at random at the
sites V, D and J gene segments are recombined
The 3rd CDR/hypervariable loop corresponds to the join
between the V and J segment and in the heavy chain is
partially coded by the D segment
Junctional diversity greatly increases the number of
antigenic specificities
Naïve B-cells that exit the bone marrow express what? (2)
IgM and IgD on their cell surface
What is the first immunoglobulin secreted in response to antigens? (1)
IgM
How does class switching occur? (2)
As the humoral response matures B-cells undergo
class switching and one of either IgG, IgA and IgE are
expressed instead of IgM/IgD by B-cells
Which region of the
heavy chain is switched? (1)
Only constant region - antigenic specificity unchanged
Genetic basis of isotype
switching (3)
B-cell DNA after VDJ recombination: makes IgM and IgD
B-cell DNA after isotype switching: IgM and IgD heavy
chains encoding regions are excised
B-cell now makes IgG, but antigenic specificity is
unaltered as the variable region encoded by VDJ
segments is unaltered
