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hi this question had a simple explanation which I just loved so I thought I'd share it. it's from the usmle-consult online q-bank which I won from here

A 62-year-old woman complains of progressive weakness, tiredness, and more frequent illness over the past year. Protein analysis of her blood yields an abundance of immunoglobulin A (IgA). Analysis of the immunoglobulin gene arrangement from the predominant cell in a bone marrow aspirate confirmed the diagnosis. Which of the following heavy chain gene arrangements were observed in these cells?

A.
Alpha
B. Epsilon, alpha
C. Gamma, epsilon, alpha
D. Mu, delta, gamma, epsilon, alpha
E. Mu, gamma, epsilon, alpha
Explanation:

Option A (Alpha) is correct. The heavy chain gene is organized as follows: mdgea. Expression of IgA requires the deletion of all heavy chain genes that precede it. IgD is expressed with the same gene arrangement as IgM and is generated by posttranscriptional modification.
 

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This is a beautiful explanation if it works - but I didn't understand the part about igD - can anyone explain? Whats on an IgD then vs. and IgM?
 

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different type of gene arrangements occur in naive B cells n after isotype switching...
in naive B cells only IgM n IgD are produced....with same gene arrangements...means VDJ-MDGEA....for both IgM n IgD...
it is the post translation modification that splice either D or M to produce IgM n IgD respectively,......

but in isotype switching.....gene arrangements occur at DNA level only....by activation n excision of switching regions...like if want to make IgG....then MD will be spliced...from MDGEA....so only GEA is left...

for IgE...MDG will be spliced....n remains only EA.....n for IgA....only A is remained...

once the cell has gone in isotype switching....it wont be able make previous Ig type again....

like if ur cell is synthesizing IgG...can't make IgM & IgD
 
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