B cell receptor

A B cell receptor (BCR) is the membrane-bound antibody molecule displayed on the surface of a B cell, serving as both sensor and identity marker. Each B cell expresses approximately 10^5 identical BCRs, all generated from the same V(D)J rearrangement event that produced the cell's unique antigen specificity. The BCR is not merely a passive recognition device; it is a signaling hub that transduces antigen binding into intracellular activation cascades, triggering proliferation, differentiation, and antibody secretion. The receptor's dual function — recognition and signal transduction — is what distinguishes B cells from all other cell types and makes them capable of initiating adaptive immune responses without external instruction.

The BCR is also the physical substrate of clonal identity. When a B cell divides, its daughter cells inherit the same receptor specificity, creating a clone unified by shared recognition capacity. This clonal unity is the molecular basis of clonal selection: the immune system does not select cells individually; it selects clones, and the BCR is the tag that marks clone membership.