Bone marrow is a spongy material that is found inside the bones (particularly the pelvic bones). Stem cells are blood cells at the earliest stage of development in the bone marrow. Within the bone marrow, stem cells develop into the different blood cells described below. When the cells are fully mature they are released into the bloodstream.

Like a factory, bone marrow produces the cells which develop into the three different types of blood cells:

• red blood cells, which carry oxygen to all cells in the body
• white blood cells, which are essential for fighting infection
• platelets, which help the blood to clot and prevent bleeding.

Normally, most of the stem cells in the body are in the bone marrow and there are only very small numbers in the bloodstream. However, it is possible to stimulate the stem cells to move into the bloodstream, by using low doses of certain chemotherapy drugs or injections of growth factors. Stem cells can be collected from the bloodstream or from the bone marrow.

A transplant using stem cells collected from the bone marrow is sometimes called a bone marrow transplant, when in fact it is really a transplant of stem cells.