B cell activation

Question 1

Mantle zone → Dark Zone transition

Answer 1

• Where B cells process and present antigen to activated CD4+ T cells
• CD40 (B cell) and CD40L (T cell) interaction [2nd signal] and cytokines [3rd signal] → activation of B cells in the mantle zone → migration of B cells into the dark zone of the germinal centers where they proliferate as centroblasts

Question 2

Follicular DCs

Answer 2

• Not real DCs or APCs b/c they don’t have MHCII
• They express non-self antigens on Fc receptors and complement (CR1) C3b receptors for the selection of germinal center B cells
  
  • They use these complement receptors to test the B cells for good binding
    
    • Only B cells with high affinity BCRs survive
    • B cells with low affinity BCRs will undergo apoptosis
• Present in the light zone of the germinal centers

Question 3

Follicular T cells

Answer 3

• Are in the light zone of the germinal centers and help the centrocytes mature
• They express CD40L and secrete lots of IL-4
• They send the signals for SHM and class-switching
• They prevent apoptosis (are the survival signal)

Question 4

What enzyme induces point mutations in DNA regions of B cells (SHM)?

Answer 4

AID is the enzyme responsible for inducing point mutations in DNA regions during **somatic hypermutation **(SHM) in B cells.

SHM occurs in the dark zone of the GCs where the centroblasts are

Question 5

What enzyme and cytokine are involved in isotype switching?

Answer 5

AID

&

IL-4

Isotype switching occurs in the light zone of the GCs, where centrocytes are

Question 6

What is the fate of memory B cells?

Answer 6

Enter the **recirculating **lymphocyte population

Question 7

What is the fate of plasma cells?

Answer 7

Leave and reside in the bone marrow

Question 8

IgM Function & Properties

Answer 8

• “First responder“/early response of humoral immunity
• Secreted before IgG
• Does not cross the placenta
• May be attached to the surface of a B cell or secreted into the blood
• Pentamer

Question 9

IgA Function, Properties, & Inducers

Answer 9

• Found in mucus, saliva, tears, and breast milk
  
  • Lines GI, respiratory, & urogenital tracts
• Dimer
• Protects against pathogens
• Induced by TGF-beta; IL-5 augments production

Question 10

IgD Function & Properties

Answer 10

• Found on naive B cells (w/ no antigen exposure)
• Part of the B cell receptor
• Activates basophils and mast cells, but function is mostly unknown

Question 11

IgG Function, Properties, Inducers, & Inhibitors

Answer 11

• Major defender of the body against invading pathogens
• Crosses the placenta → involved in maternal-fetal immunity
• Secreted by plasma cells in the blood
• Induced by IL-4
• Inhibited by IFN-gamma

Question 12

IgE Function, Properties, Inducers, & Inhibitors

Answer 12

• Binds allergens and parasitic worms
• Causes mast cells and basophils to release histamine
• Responsible for allergic reactions
• Induced by IL-4
• Inhibited* by *INF-gamma

Question 13

Advantages of T cell dependent B cell responses

Answer 13

TD B cell responses require T cell costimulation

• Somatic hypermutation (SHM)
• Class switching
• Plasma cells that produce high affiniy antibodies
• Memory B cells
• Long-lasting (IgG, IgA, & IgE)

Occurs in the **dark zone **of the germinal centers

Question 14

Disadvantages of T cell independent B cell responses

Answer 14

B cells are activated via crosslinking of BCRs w/ antigenic peptides

• No class-switching
• No plasma cells
• No memory
• Mostly short-lasting (IgM)

Occurs in the mantle zone of the lymph nodes

Question 15

Dark Zone → Light Zone transition

Answer 15

• The centroblasts that have proliferated in the dark zone have undergone SHM that has produced BCRs that have ⇡ or ⇣ affinity
• Now the centroblasts migrate into the light zone where they become centrocytes and their affinity is tested by the follicular DCs there