In human anatomy, red bone marrow is the primary site of new blood-cell production after birth.
This process, called haematopoiesis, produces red blood cells, most white blood cells and platelets from multipotent haematopoietic stem cells. Red marrow contains a specialised network of developing cells, supporting stromal cells and blood vessels that release mature cells into circulation.
In children, red marrow is found throughout much of the skeleton. During growth, much of it changes into yellow marrow, which stores fat. In healthy adults, active red marrow is concentrated mainly in the pelvis, vertebrae, ribs, sternum and the ends of some long bones.
The spleen and thymus have important immune roles but are not the main source of all new blood cells after birth. The thymus supports T-cell maturation, while the spleen filters blood and helps remove aged cells. Severe marrow disorders can disrupt several blood-cell lines at once, causing anaemia, infection risk or abnormal bleeding.