Q.43 B cells produce two forms of an immunoglobulin: (i) membrane–bound form,
known as B cell receptor (BCR) and (ii) soluble form, known as antibody. Which
of the following statements is/are CORRECT about BCR and antibody produced by
the same B cell?
(A) BCR and antibody have identical antigen binding site
(B) BCR and antibody recognize different epitopes
(C) BCR and antibody are encoded by the same gene
(D) BCR and antibody are formed by differential splicing

B cells produce both membrane-bound BCR and soluble antibodies from the same immunoglobulin gene through alternative RNA processing.

Correct Answer: (A), (C), and (D)

Option Analysis

Option (A): BCR and antibody have identical antigen binding site
This statement is correct. The BCR and secreted antibody from the same B cell share identical variable regions (VH and VL domains), forming the same antigen-binding site for specific epitope recognition.​

Option (B): BCR and antibody recognize different epitopes
This statement is incorrect. Both forms recognize the identical epitope due to the same variable regions generated during V(D)J recombination.​

Option (C): BCR and antibody are encoded by the same gene
This statement is correct. A single rearranged immunoglobulin heavy chain gene encodes both forms, with differences arising from post-transcriptional processing.​

Option (D): BCR and antibody are formed by differential splicing
This statement is correct. The heavy chain pre-mRNA undergoes alternative splicing and polyadenylation: secreted form uses secretory exon (polyA site upstream), while membrane-bound BCR includes transmembrane exons (M1/M2) downstream.​

B cells produce two forms of immunoglobulin: membrane-bound B cell receptor (BCR) and soluble antibody, both critical for adaptive immunity. These forms share identical antigen binding sites but differ in structure and function due to differential splicing of the same gene.​

BCR and Antibody Structure

The BCR anchors to the B cell membrane via transmembrane domains in its heavy chain, associating with Igα/Igβ for signaling. In contrast, the soluble antibody lacks these domains, featuring a secretory tail for circulation and effector functions like pathogen neutralization.​
Variable regions remain identical, ensuring the same epitope specificity.​

Genetic Mechanism: Differential Splicing

A single heavy chain gene transcript contains exons for variable/constant domains, plus alternative 3′ ends.

• Secreted form: Splicing excludes membrane exons; polyadenylation at secretory site.

• Membrane-bound BCR: Includes M1/M2 exons for anchor; polyadenylation downstream.​
  This allows naive B cells to express BCR for antigen detection and plasma cells to secrete antibodies.​

Functional Differences

This table highlights key BCR vs antibody distinctions vital for CSIR NET Life Sciences preparation.​