The marrow inside your bones is an active tissue with a rich blood supply. Your bone marrow produces most of your blood cells, including all of the red cells and platelets, and most of the white cells.

The blood that flows through the marrow moves the blood cells that the marrow produces into the bloodstream. In an adult, active blood-forming marrow is only in the bones of the body’s trunk. The limb bones contain fatty, non-active marrow, which can change to active, blood-forming marrow if the body needs to produce more blood cells. In a young child, all the bones have active blood-forming marrow.

This section includes diseases in which the bone marrow produces either too many or too few blood cells. These disorders may affect all types of blood cells, as in plastic anemia and polycythemia, or may only affect one type of cell, as in multiple myeloma and agranulocytosis.