Lecture 14. Class Switching and Generation of Diversity and Memory

Question 1

What are the N-terminal domains of both chains in an antibody called?

Answer 1

‘Variable’ (V) domains

Question 2

What are the non variable domains called in an antibody?

Answer 2

‘Constant’ (C) domains

Question 3

What does IgM contain?

Answer 3

Multiple disulfide bonds, and the H chain is heavily N-glycosylated

Question 4

What are N-glycans?

Answer 4

Complex carbohydrates that are added to asparagine residues during folding prior to secretion: they are large and so they hold domains apart, allowing exposure of functional motifs (e.g. complement binding sites)

Question 5

What is the function of the Cγ1 and Cγ2 domains in IgG?

Answer 5

Bind complement components

Question 6

What is the function of the Cγ2 and Cγ3 domains in IgG?

Answer 6

Bind Fc receptors on neutropils

Question 7

What is the function of the Cγ3 domain in IgG?

Answer 7

Binds Fc receptor on macrophages and NK

Question 8

What do the multiple N-glycans in IgE result in?

Answer 8

They make IgE a stiff rigid molecule: good for targeting large pathogens, but it cannot cross-link small targets

Question 9

What does the flexible variable domain result in IgA?

Answer 9

Makes IgA a good cross-linker

Question 10

What do pre B cells in the bone marrow express?

Answer 10

IgM, membrane-bound

Question 11

What do B cells express during maturation?

Answer 11

IgM and IgD, membrane bound in lymphoid tissue

Question 12

What is the antigen binding site made up of?

Answer 12

VL and VH domain interactions

Question 13

How does class switching occur in IgM?

Answer 13

There is one antibody H chain gene, with an unusual arrangement
The Ig H chain gene encodes a variable (VH) domain and all the H chain constant regions, separated by non-coding introns
Class switching occurs through use of somatic recombination of DNA

Question 14

Process of class switching

Answer 14

The genomic DNA is looped
An unusual DNA recombination event can now occur, between sequences called switch regions
This requires specialised sets of proteins
Cutting and rejoining of DNA results in excision of the loop: the result is class switching

Question 15

What is clonal selection?

Answer 15

Following infection, individual clones are selected by antigen, based on how well the antigen and the receptor fit together: the result is pathogen-specific lymphocytes are selected from pools of B and T cells.

Question 16

What is clonal expansion?

Answer 16

The selected clones undergo mitosis, proliferate and differentiate into effector cells

Question 17

What is clonal deletion?

Answer 17

Those lymphocytes that react inappropriately with ‘self’ antigens are destroyed

Question 18

How does primary antibody diversity occur?

Answer 18

1. There are only 3 antibody genes. There are two classes of light chains, which increases diversity
2. There are multiple gene segments encoding V domains that can be combined with C domains by somatic recombination
3. The somatic recombination (called V(D)J recombination) used to select a V gene segment is particularly complex and sophisticated and involves selection also of small pieces of ‘diversity’ and ‘joining’ DNA that ultimately link the V domain with the constant domains at the protein level

Question 19

How many different VH:VL combinations are there?

Answer 19

About 4,500

Question 20

How many unique potential antigen binding sites in proteins are there?

Answer 20

> 10¹⁴

Question 21

What is affinity maturation?

Answer 21

Over time, the antibodies made by B cells improve in affinity and become more specific

Question 22

What causes affinity maturation?

Answer 22

Accumulation of point mutations in the V domains long after the coding sequences have been assembled from the segmented genes

Question 23

Where does affinity maturation occur?

Answer 23

In the lymph nodes

Question 24

The process of affinity maturation?

Answer 24

1. Antigen stimulation causes activation and clonal expansion of B cells with pre-existing fits to the antigen, from a pool of 10¹² B cells that are potentially capable of generating 10¹⁴ unique V domain combinations
2. Some B cells of the expanded clonal population proliferate in germinal centres and undergo somatic hypermutation, generating antibodies with altered V domain specificity
3. Most of these hypermutated clones are worse than the original, and will not be stimulated by the original antigen: most will die but the rare B cells with mutated BCR versions that have higher affinity for the original antigen will proliferate

Question 25

Why is the secondary response greater and more specific than the primary response?

Answer 25

It is dominated by class-switched antibodies that have undergone somatic hypermutation

Question 26

How is immunological memory generated?

Answer 26

By the primary response

Question 27

What is the current model of T memory cells?

Answer 27

Multiple classes of memory T cells exist: some carry cell-surface markers characteristic of TH cells, others carry cell-surface markers characteristic of TC cells