B cells are a type of white blood cell that makes antibodies. B cells are part of the immune system and develop from stem cells in the bone marrow, also called B lymphocytes. Mantle cell lymphoma (MCL) starts in the B cells found in the mantle zone within a lymph node. In MCL, a B lymphocyte undergoes a change and becomes mutated, or damaged. This cancerous cell then multiplies and grows out of control, crowding out healthy cells and allowing the cancer to spread.