53. The mechanism that permits immunoglobulins to be synthesized either in a membrane bound or secreted
form is:
1. Allelic exclusion
2 Co-dominant expression
3. Differential RNA processing
4. Class-switch recombination
How Immunoglobulins Are Synthesized as Membrane-Bound or Secreted Forms
Immunoglobulins (Ig), or antibodies, are versatile proteins produced by B lymphocytes that play a vital role in the immune response. Interestingly, these molecules can exist in two distinct forms:
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A membrane-bound form that acts as the B-cell receptor (BCR)
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A secreted form that circulates in the bloodstream to neutralize pathogens
This dual functionality raises the question: How can a single gene give rise to two structurally and functionally distinct proteins?
The Correct Answer: Differential RNA Processing
The mechanism that permits this functional diversity is Differential RNA Processing.
What is Differential RNA Processing?
It refers to alternative splicing of pre-mRNA transcripts, a post-transcriptional modification that enables a single gene to produce multiple protein isoforms by including or excluding specific exons.
How It Works in Immunoglobulin Synthesis
In B cells, the immunoglobulin heavy chain gene includes exons that encode:
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The variable and constant regions (shared by both forms)
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Two alternative exons at the 3′ end — one for the membrane anchor and another for the secretory tail
The choice between membrane-bound and secreted forms is determined by:
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The pattern of RNA splicing
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The site of polyadenylation
Membrane-Bound Form:
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RNA is spliced to include the membrane-spanning exon
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Polyadenylation occurs after the membrane anchor exon
Secreted Form:
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RNA is spliced to exclude the membrane exon
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Polyadenylation occurs earlier, after the secretory exon
This elegant mechanism enables stage-specific expression of immunoglobulins — membrane-bound during B-cell development and secreted after activation.
Other Options Explained
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Allelic exclusion: Ensures that only one allele of the Ig gene is expressed in each B cell — relates to specificity, not form.
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Co-dominant expression: Refers to expression of both alleles (e.g., MHC molecules), not alternative forms.
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Class-switch recombination: Alters the isotype (IgM to IgG, etc.), not the membrane/secreted status.
Conclusion
The reason B cells can produce both membrane-bound and secreted immunoglobulins lies in the differential RNA processing of immunoglobulin heavy chain transcripts. This allows the immune system to fine-tune antibody functions according to developmental and environmental cues.
Correct Answer: 3. Differential RNA processing
