Leukemia is cancer that affects tissues that produce blood. What is the primary site where blood cells are produced?

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

Answer: (C)

Blood cell production, or hematopoiesis, happens primarily in the bone marrow. In adults, hematopoietic stem cells reside in red bone marrow tucked into areas like the pelvis, sternum, vertebrae, ribs, and ends of long bones. These stem cells differentiate into red blood cells, white blood cells, and platelets, supplying circulating blood components. The thymus mainly matures T lymphocytes, and the spleen helps filter blood and store cells rather than being the primary production site. Lymph nodes are centers for immune activation. In fetal development, the liver and spleen contribute to hematopoiesis, but after birth the bone marrow remains the main source. In leukemia, malignant changes typically begin in the bone marrow, affecting the production of blood cells.