In which tissues are B cells matured?

B cells mature primarily in the bone marrow. This is a critical process where precursor B cells undergo several developmental stages, including heavy and light chain gene rearrangement, ultimately leading to the expression of B cell receptors (BCRs) on their surface. Successful maturation allows B cells to exit the bone marrow as naive B cells, which can subsequently enter the bloodstream and move to peripheral tissues such as lymph nodes and the spleen.

In contrast, the thymus is the site of T cell maturation, not B cells. Lymph nodes and the spleen play important roles in the activation and proliferation of B cells after they have matured and encountered antigens, but they are not sites where B cells mature. Thus, the correct identification of the bone marrow as the maturation site emphasizes the specific development pathway for B cells before they can become fully functional immune cells.