B cell activation Flashcards
Mantle zone → Dark Zone transition
- 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
Follicular DCs
- 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
- They use these complement receptors to test the B cells for good binding
- Present in the light zone of the germinal centers
Follicular T cells
- 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)
What enzyme induces point mutations in DNA regions of B cells (SHM)?
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
What enzyme and cytokine are involved in isotype switching?
AID
&
IL-4
Isotype switching occurs in the light zone of the GCs, where centrocytes are
What is the fate of memory B cells?
Enter the **recirculating **lymphocyte population
What is the fate of plasma cells?
Leave and reside in the bone marrow
IgM Function & Properties
- “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
IgA Function, Properties, & Inducers
- 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
IgD Function & Properties
- 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
IgG Function, Properties, Inducers, & Inhibitors
- 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
IgE Function, Properties, Inducers, & Inhibitors
- 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
Advantages of T cell dependent B cell responses
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
Disadvantages of T cell independent B cell responses
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
Dark Zone → Light Zone transition
- 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
