Lecture 22- somatic hypermutation, affinity maturation and class- switching in B cells.

Question 1

What is the result of CD40 signalling from the Tfh cell to the B cell?

Answer 1

• B cell proliferation
• expression of activation-induced cytosine deaminase (AID)

Question 2

What is the function of AID?

Answer 2

1. somatic hypermutation (point)
2. Class (or isotype) switching. (excision of DNA)

Question 3

Where does somatic hypermutation occur? Where does class switching occur?

Answer 3

1. dark zone during proliferation
2. light zone

Question 4

How does class switching differ from somatic recombination?

Answer 4

similar process, still excise DNA, but no VDJ recombination

Question 5

What is somatic hypermutation?

Answer 5

AID is a DNA deaminase, and removes an amino group from cytosines (C) in a single stranded template and converts it to deaminated uracil (U)

Question 6

When are hypermutations introduced to the DNA?

Answer 6

When the DNA is being replicated

Question 7

How many rounds of DNA replication does it take for the mutation to be stabilized?

Answer 7

2 rounds

Question 8

What is the function of UNG? (uracil-DNA glycosylase)

Answer 8

excise the uracil base to create an abasic site in the DNA (take out the base for DNA repair processes to make a substitution)

Question 9

Why is antigen only present in low amounts in the germinal centre?

Answer 9

It is being consumed by FDCs

Question 10

Can the B cell go back and forth between the dark and light zone for hypermutation?

Answer 10

yes

Question 11

Where does most of the isotype switching occur (specifically)?

Answer 11

germinal centres

Question 12

What is isotype switching?

Answer 12

Involves only the heavy chain genes and generates antibodies
of different classes; the different classes of antibodies perform distinct effector
functions. The specificity of the antibody remains the same.

Question 13

What regulates isotype switching?

Answer 13

Th cytokines