Where are blood cells created?

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

Answer: (B)

Blood cell production, or hematopoiesis, occurs primarily in bone marrow. Within the marrow, hematopoietic stem cells differentiate into all the different blood cell types—red blood cells, various white blood cells, and platelets—under the influence of growth factors and the supportive marrow environment. In adults, the marrow in certain bones serves as the main factory for this ongoing production.

The other organs listed have important roles in blood and immune function but are not the regular sites of new blood cell creation. The spleen helps filter blood and can store platelets, and the liver is essential during fetal development and can contribute under certain disease conditions, but normal adult blood cell formation mainly happens in bone marrow. Lymph nodes are centers for immune cell activation and maturation, not for producing blood cells.