Antibody Diversity and Antibody Class Switching

Question 1

What is antibody diversity? (1)

Answer 1

Refers to the array of different antigenic specificities produced in the humoral immune response

Question 2

Antigenic specificity of an antibody is dictated by what? (1)

Answer 2

Antigenic specificity of an antibody is dictated by the sequence of the VL and VH domains

Question 3

How does antigenic specificity arise? (2)

Answer 3

Generated by recombination of the antibody genes during the antigen-independent differentiation of B-cells in the bone marrow

Question 4

How much antibody diversity is produced? (3)

Answer 4

~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

Question 5

How many antibody genes are there? (2)

Answer 5

We have 3 antibody genes 2 light chain (kappa and lambda) and 1 heavy chain gene

Question 6

How can we make antibodies with ~5 X 1013 different specificities when there are only 3 antibody genes? (1)

Answer 6

Diversity is achieved by recombination of less than 1,000 gene segments in the DNA of developing B-cells

Question 7

When is diversity of antibody genes generated? (1)

Answer 7

During the antigen independent development of B-cells

Question 8

How does recombination of gene segments cause antibody specificity for antigen? (3)

Answer 8

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

Question 9

How is the coding region of the variable domain of the light chain generated? (2)

Answer 9

Each light chain uses one V, and one J gene segment to generate the coding region for the variable domain

Question 10

How is the coding region of the variable domain of the heavy chain generated? (2)

Answer 10

Each heavy chain uses one V, one D and one J gene segment to generate the coding region for the variable domain

Question 11

Which process generates the coding region for the variable domain? (1)

Answer 11

V (D) J recombination

Question 12

Where are heavy chains associated with light chain in a cell? (1)

Answer 12

Heavy chains associate with light chains in the ER to make antibody molecules

Question 13

What is combinatorial diversity? (2)

Answer 13

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

Question 14

How does antigen-independent B cell differentiation occur in the bone marrow? (4)

Answer 14

Lymphoid progenitor&raquo_space;> Recombination of
immunoglobulin genes&raquo_space;> Expression of antibody surface receptors (IgD and IgM)&raquo_space;> B-cells then migrate to secondary lymphoid tissues

Question 15

Lifetime of B cell

Answer 15

B-cells migrate to secondary lymphoid tissues but will die if they do not recognise specific antigen within ~ 21 days

Question 16

What does class switching do? (3)

Answer 16

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

Question 17

Immunoglobulin genes (2)

Answer 17

Immunoglobulin genes are non-functional until
they are rearranged

Immunoglobulin genes are rearranged during
antigen independent differentiation of B-cells
in the bone marrow

Question 18

Describe the basic principle of light chain gene recombination (7)

Answer 18

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

Question 19

Describe the basic principle of heavy chain gene recombination (7)

Answer 19

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

Question 20

Arithmetic of VDJ recombination

Answer 20

CHECK LECTURE SLIDE

Question 21

Describe junctional diversity (4)

Answer 21

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

Question 22

Naïve B-cells that exit the bone marrow express what? (2)

Answer 22

IgM and IgD on their cell surface

Question 23

What is the first immunoglobulin secreted in response to antigens? (1)

Answer 23

IgM

Question 24

How does class switching occur? (2)

Answer 24

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

Question 25

Which region of the
heavy chain is switched? (1)

Answer 25

Only constant region - antigenic specificity unchanged

Question 26

Genetic basis of isotype
switching (3)

Answer 26

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