Ch. 12 B Cell Activation

Question 1

What are iccosomes?

Answer 1

FDC dendrites are studded with antigen-antibody complexes

Question 2

Are iccosomes more helpful in activation of naive cells or in CSR and SHM activities that allow B cell differentiation?

Answer 2

Aid in later differentiation of B cells, and not necessarily primary activation (CSR and SHM)
-B cells bind to antigens to prove they are the best

Question 3

In what ways can a B cell acquire and engulf its cognate antigen?

Answer 3

• lysosomal fusion results in proteolytic enzymes cleaving bonds between antigen and APC
• a more “muscular” response which involve BCR tugging facilitated by actomyosin rearrangements

Question 4

What transcription factors dictate whether a B cell will enter a primary focus or a germinal center?

Answer 4

• IRF-4 and BLIMP-1 trigger primary foci plasma cell differentiation
• Pax-5 and Bcl-6 trigger germinal center formation

Question 5

How many days before primary foci can be observed following antigen exposure?

Answer 5

Plasma cells in the primary focus are generally found within 5-6 days of exposure in the medullary cord region of the lymph node and then die by apoptosis 5-10 days later

Question 6

How many days before there is a peak in primary foci activity?

Answer 6

7-8 days after antigen encounter

Question 7

How long do plasma cells tend to live before dying by apoptosis (in primary foci)?

Answer 7

5-10 days after they are found

Question 8

What occurs in germinal centeers?

Answer 8

SHM and CSR

Question 9

Where would you expect to find germinal centers in a lymph node?

Answer 9

within the follicles

Question 10

What is occurring in the dark zone of a germinal center?

Answer 10

densely packed with intensely proliferating B cells (SHM)

Question 11

What is occurring in the light zone of a germinal center?

Answer 11

B cells interspersed with a network of FDCs (trying to grab all the antigen they can)

Question 12

What is Class Switch Recombination?

Answer 12

The m constant region gene segment is switched with other constant region gene segments to allow for B cells to secrete antibodies other than IgM.

Question 13

How does AID lead to SHM?

Answer 13

Produces opportunity for mutation by altering cytidine bases to form uridine bases, which can cause a variety of mutations

Question 14

What are the 3 outcomes to AID action?

Answer 14

1. UG is not a natural pair, the “U” is read as a “T” by the DNA replication machinery
2. Short-patch base excision repair using error-prone polymerases causes a point mutation
3. Mismatch repair excises a short stretch of base pairs, which are repaired by error-prone polymerases

Question 15

How does the AID enzyme know how to act on certain areas of the genome?

Answer 15

• restricted to Ig gene segments

- Mutational apparatus targeting

Question 16

Over time, B cells in germinal centers have a higher affinity than they started with. How is this accomplished?

Answer 16

• point of SHM is to create B cells that bind strongly to the new antigen
• affinity selections tests occur
• cells that can bind, process, and present more Ag to T cells for cytokine assistance survive

Question 17

What happens if a B cell accidentally creates an autoreactive BCR during SHM?

Answer 17

1. B cells that develop receptors for self-antigen are overwhelmed by high concentrations of self-antigen in lymph nodes
   - rapid internalization of large percentage of surface BCRs
   - As almost all surface BCR will be lost, BCR signaling is lost –> exits cell and dies from apoptosis
2. Anergy