B-cell receptor on naive B lymphocytes: Which immunoglobulin is considered the primary surface receptor on mature naive B cells in peripheral lymphoid tissues?

Introduction / Context:
B cells display membrane-bound immunoglobulin as their antigen receptor (BCR). During development and in the mature naive state, different isotypes can be expressed on the surface. This question probes which isotype is recognized as the primary B-cell receptor on naive B cells circulating in lymphoid organs.

Given Data / Assumptions:

• Naive mature B cells co-express surface IgM and IgD due to alternative RNA splicing of a single heavy-chain transcript.
• Both isotypes can bind antigen and transduce signals through Ig-alpha/Ig-beta (CD79a/CD79b).
• Many curricula emphasize IgD as the hallmark BCR on naive mature B cells in secondary lymphoid tissues.

Concept / Approach:
Although immature B cells initially express IgM, upon maturation they co-express IgM and IgD. In textbooks and exam settings, IgD is often highlighted as the characteristic primary receptor on naive mature B cells in the periphery, while IgM remains present but is more associated with earlier stages and is the first antibody produced upon initial activation.

Step-by-Step Solution:

Verification / Alternative check:
Flow cytometry of mature naive B cells shows strong IgD and IgM; exam conventions typically accept IgD as the defining BCR for naive cells in lymph nodes and spleen.

Why Other Options Are Wrong:

Common Pitfalls:
Ignoring that naive B cells co-express both IgM and IgD; overgeneralizing “first made is IgM” to the identity of the mature naive BCR in lymphoid tissues.

Final Answer:
IgD (membrane-bound).