Blood cells consist of red cells, white cells, and platelets, and are made in the bone marrow, a soft spongy tissue found in the center of large bones. The red cells carry oxygen throughout the body, white cells fight infections, and platelets clot the blood when there is an injury. The body carefully regulates the production of these cells.

If the regulation process is disrupted, a blood disorder can occur. Hematologists are doctors who specialize in blood disorders.