Stem cells are cells in the bone marrow that make all of the body's blood cells. Stem cell transplants are used to restore the stem cells when the bone marrow has been destroyed by disease, chemotherapy (chemo), or radiation. Depending on the source of the stem cells, this procedure may be called a bone marrow transplant, a peripheral blood stem cell transplant, or a cord blood transplant. We will give you more detail on each of these later.

The first successful bone marrow transplant was done in 1968. It was not until nearly 20 years later that stem cells taken from circulating (peripheral) blood were transplanted with success. More recently, doctors have begun using cord blood from the placenta and umbilical cords of newborn babies as another source of stem cells.

Today tens of thousands of patients have had stem cell transplants. This has led to better care for transplant patients and helped doctors know more about which patients are likely to have better results after transplant.

What are stem cells?

All of the blood cells in our bodies start out as young (immature) cells called hematopoietic stem cells. (Hematopoietic means blood-forming.) Even though they are often called "stem cells" these cells are not the same as the embryonic stem cells studied in cloning and other types of research.

Stem cells mostly live in the bone marrow (the spongy inner part of certain bones), where they divide to make new blood cells. Once blood cells are mature enough they leave the bone marrow and enter the bloodstream. Still, a small number of stem cells are also found in the bloodstream. These are called peripheral blood stem cells.

Stem cells make the 3 main types of blood cells:

• red blood cells
• white blood cells
• platelets

Red blood cells (erythrocytes)

Red blood cells (RBCs) carry oxygen from the lungs to all of the cells in the body, and then bring carbon dioxide back from the cells to the lungs to be exhaled. A lab test of the blood called the hematocrit shows how much of your blood is made up of RBCs. The normal range is about 35% to 50% for adults. People whose hematocrit is below this level have anemia. This can make them look pale and feel cold, tired, and short of breath.

White blood cells (leukocytes)

White blood cells (WBCs) fight infections caused by bacteria, viruses, and fungi. There are different types of WBCs. The most important in fighting infections are called neutrophils. When your absolute neutrophil count (ANC) drops below 1,000 per cubic millimeter (1,000/mm³) you have neutropenia, and your risk of infection increases. The danger is greatest at levels below 500/mm³.

Stem cells also make white blood cells called lymphocytes. These are immune cells that can make antibodies and help fight infections. They are also responsible for a person's ability to reject cells that are transplanted from someone else.

Platelets (thrombocytes)

Platelets are pieces of cells that seal damaged blood vessels and help blood to clot, both of which are important in stopping bleeding. A normal platelet count is usually between 150,000/mm³ and 450,000/mm³. A person whose platelet count drops below 150,000/mm³ is said to have thrombocytopenia, and may bruise more easily, bleed longer, and have nosebleeds or bleeding gums. Spontaneous bleeding (bleeding with no known injury) can happen if a person's platelet count drops lower than 20,000/mm³.

More information on blood counts and what the numbers on your test results may mean is available in Understanding Your Lab Test Results which you can get by calling 1-800-227-2345.

Last Medical Review: 05/27/2009
Last Revised: 05/27/2009