Question

24. Which of the following accounts for the transition from membrane-bound IgM to secreted IgM upon activation of a B cell?

A. CSR
B. Affinity maturation
C. Alternative splicing of pre-mRNA
D. Additional modification of nuclear DNA via RAG-1/2
E. None of the above

Answer 1

The transition from membrane-bound IgM to secreted IgM upon activation of a B cell is accounted for by alternative splicing of pre-mRNA.

Step-by-step explanation:

Upon activation of a B cell, the transition from membrane-bound IgM to secreted IgM occurs through a process called class switching or isotype switching. This process involves the genetic rearrangement of gene segments encoding the constant region of the antibody, while the variable region remains unchanged. The production of secreted IgM allows plasma cells derived from the activated B cell to secrete large quantities of IgM antibodies into the blood.

The question asks which mechanism accounts for the transition from membrane-bound IgM to secreted IgM upon activation of a B cell. The correct answer is C. Alternative splicing of pre-mRNA. B cells initially produce membrane-bound IgM that functions as antigen receptors.

Upon activation, these cells can undergo a process that results in the secretion of IgM antibodies. Alternative splicing of the heavy chain mRNA for IgM allows a single gene to produce two different forms of the protein: one that is membrane-bound and another that can be secreted. This post-transcriptional modification allows for the production of different antibodies serving their respective functions without altering the specificity of the antigen.