immuno q from NBME 6 someone help plz

[sadaca]

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A 20 y.ol woman with difficulty breathing for 2 hrs. Overdistended chest marked restriction of expansion. Increased blood eosinophils and eosinophils in sputum. Which is most likely sequence of events for initial sensitization of to allergen causing these findings?

A- cytokine-----IgE-------T-Lymph induction
B- IgE------B lymp------T lymph
C- IgE------T lymph------cytokine
D- T-------cytokine-------IgE

ans is D checked several blogs..and I thought I knew this and heres how I know (from kaplan book) BUT after learning it the way mentioned below it leads to B b/c u need class switching or even C b/c shouldnt release of mediators from mast cell be the last step?
someone plzz explain this, I want to get this right next time. REALLy appreciate it

Upon exposure to allergen for first time,
1....IgE antibody production (sensitization)
Allergens (e.g., pollen, drugs) are first processed by APCs (macrophages or dendritic cells).
APCs interact with CD4 TH2 cells, causing interleukins (ILs) to stimulate B-cell maturation.
IL-4 causes plasma cells to switch from IgM to IgE synthesis.
IL-5 stimulates the production and activation of eosinophils.


2...
Mast cell activation (reexposure)
Allergen-specific IgE antibodies are bound to mast cells.
Allergens cross-link IgE antibodies specific for the allergen on mast cell membranes.

 

[notbobtrustme]

need IL-4 + IL-17 secretion by Th2 cells to induce IgE class switching in B cells. Th2 cell was activated when it encountered the antigen and secreted IL-4 and 17 in response to various factors and contexts (not all of it has been worked out yet). This caused B cells sensitive to that same antigen to class switch to IgE and differentiate to plasma cells. Those plasma cells then secreted IgE, which was bound on mast cells, and trigged degranulation when the antigen was encountered again. Hope that makes sense.

 

[sadaca]

ok thank you . I am still not sure how that leads to the answer choice B especially since you also said you have to have B cells to undergo class switching.. could u expand on how to get the answer from the above q plz

 

[alicealicealice]

It is the INITIAL sensitization.
Think about it: You want to put B or C as your answer and they both start with IgE. What the question is asking you is how the IgE got there in the first place. The INITIAL sensitization is all about how you get the IgE made and get the mast cells primed for the NEXT exposure, which is you when you first get actual symptoms. The initial sensitization stage is not associated with any symptoms.

So the INITIAL sensitization goes like this: Allergen binds to Immature B cells and is presented to CD4 Helper T cells. We're setting up a type I HSR reaction here which is purely humoral. So the body creates Th2 helpers, which secrete IL-4 (cytokines) onto the B cells, which induces a class switch to IgE. (Choice D)

Now IgE is made and is hooked on to mast cells. Nothings really happened yet clinically. You are sensitized. On RE-EXPOSURE, then you get antigen binding to IgE on mast cells and degranulation etc etc. This is not what the question is asking.

Furthermore, B and C don't really make sense because they're backwards: No immune process starts with an Ig and then activates B Cells or T Cells. T cells and B cells work together to make Ig. Same thing with choice A.

 

[sadaca]

thank you soo much alice