Which organ houses the progenitors that give rise to B lymphocytes?

• A. Thymus
• B. Bone marrow
• C. Spleen
• D. Lymph node

Answer: (B)

B lymphocyte progenitors originate and mature in the bone marrow. Hematopoietic stem cells give rise to common lymphoid progenitors that commit to the B cell lineage and progress through pro-B, pre-B, and immature B stages within the marrow. This development relies on the marrow microenvironment and signals like IL-7 from stromal cells. Once B cells reach the immature stage and express surface IgM, they exit to the periphery as naive B cells and seed secondary lymphoid tissues, such as the spleen and lymph nodes, where they can encounter antigen and become activated.

The thymus is where T cell precursors mature, not B cell progenitors, while the spleen and lymph nodes are sites for mature B cell residence and immune activation rather than primary sites of B cell development.