An immature B cell in the bone marrow has just assembled a BCR through V(D)J recombination. When tested against bone marrow self-antigens, its receptor binds strongly to one. What is the cell's most likely first response?
AImmediate apoptosis to eliminate the dangerous self-reactive clone
BReceptor editing — reactivating the recombination machinery to try a new light chain
CRelease into circulation in an anergic (functionally silenced) state
DActivation and proliferation against the self-antigen
Receptor editing is the primary first response to self-reactivity: the cell reactivates its V(D)J recombination machinery and attempts to rearrange a different light chain, hoping to generate a non-self-reactive BCR. Only if editing fails does the cell undergo clonal deletion (apoptosis) or anergy. The common misconception is that self-reactive cells are simply killed immediately, but receptor editing gives them a second chance — this matters because it recovers cells that happen to have functional heavy chains but problematic light chains.
Question 2 Multiple Choice
V(D)J recombination generates BCR diversity primarily through which mechanism?
ASomatic hypermutation of the variable region in bone marrow germinal centers
BCombinatorial joining of V, D, and J gene segments plus junctional diversity at the splice sites
CSelection of pre-formed BCRs from a genomically encoded library of antigen specificities
DClass switching to different immunoglobulin isotypes during development
V(D)J recombination assembles each BCR from randomly selected V, D, and J gene segments (heavy chain) or V and J segments (light chain), with additional junctional diversity from imprecise joining. This somatic DNA rearrangement occurs in the bone marrow before antigen exposure. Somatic hypermutation — option A — is a different process that refines affinity AFTER antigen encounter in germinal centers. Class switching (option D) also happens post-activation and changes the antibody isotype, not the antigen-binding specificity.
Question 3 True / False
Mature naive B cells express both IgM and IgD on their surface when they exit the bone marrow.
TTrue
FFalse
Answer: True
Co-expression of surface IgM and IgD is the hallmark of a fully mature naive B cell. Both isotypes carry the same antigen-binding variable region (same BCR specificity) but different constant regions, produced by alternative splicing of the same heavy chain mRNA. The IgD may help tune the activation threshold. This dual expression is what distinguishes a mature naive B cell from an immature B cell, which expresses only IgM.
Question 4 True / False
Any immature B cell that binds a self-antigen in the bone marrow will be eliminated by apoptosis — this is how central tolerance maintains self-tolerance.
TTrue
FFalse
Answer: False
Central tolerance operates through three possible fates for self-reactive immature B cells, not just one. Receptor editing (rearranging a new light chain) is attempted first. Clonal deletion (apoptosis) follows if editing fails. Anergy (functional silencing) can also occur, allowing the cell to survive but leaving it unable to respond. Apoptosis is not the universal or even primary outcome — receptor editing rescues a substantial fraction of initially self-reactive cells.
Question 5 Short Answer
Why must B cells undergo a central tolerance checkpoint in the bone marrow if V(D)J recombination is what generates their diversity?
Think about your answer, then reveal below.
Model answer: Because V(D)J recombination is random — it assembles gene segments without regard to what the resulting BCR will recognize. A fraction of randomly generated receptors will, by chance, bind to the body's own molecules. Without a tolerance checkpoint, these self-reactive clones would be released and could drive autoimmune responses. Central tolerance is the necessary consequence of using a random combinatorial diversity mechanism: you get huge diversity, but you must then filter out the clones that happen to be dangerous.
The randomness of V(D)J recombination is both the strength and the liability of the adaptive immune system. Any system that generates ~10^11 potential specificities at random will inevitably produce many that recognize self. Central tolerance in the bone marrow is the quality-control step that makes the system viable — it is not an add-on but a logical necessity of the diversity mechanism. This is why disruption of central tolerance checkpoints leads to systemic autoimmune diseases.