- The primary development of B lymphocytes occurs in the bone marrow.
- B lymphocytes, or B cells, are a crucial part of the adaptive immune system, responsible for producing antibodies.
- During their development in the bone marrow, B cells undergo processes such as gene rearrangement of their immunoglobulin genes, which is essential for creating a diverse antibody repertoire.

In contrast, the thymus is the primary site for T lymphocyte development, where immature T cells mature and undergo selection processes. The spleen and lymph nodes are secondary lymphoid organs, which serve as sites for antigen exposure and activation of mature lymphocytes but are not involved in their primary development.

Key points:
- Bone marrow: Primary site for B cell development and maturation.
- Thymus: Primary site for T cell development, not B cells.
- Spleen and lymph nodes: Secondary lymphoid organs, sites for activation of mature lymphocytes.
- B cell maturation: Involves gene rearrangement for antibody diversity within the bone marrow.

Reference: Janeway's Immunobiology, 9th Edition, Chapter 4: Lymphocytes and Their Development