All of the blood cells in your body start out as young (immature) cells called hematopoietic stem cells. (Hematopoietic means blood-forming.) Even though they may be called “stem cells” for short, these cells are not the same as stem cells from embryos that are studied in cloning and other types of research. Here, we will use “stem cells” to mean hematopoietic stem cells.
Stem cells mostly live in the bone marrow (the spongy center of certain bones), where they divide to make new blood cells. Once blood cells are mature they leave the bone marrow and enter the bloodstream. A small number of stem cells also get into the bloodstream. These are called peripheral blood stem 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. Any of these types may be called a hematopoietic stem cell transplant.
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 hundreds of thousands of patients have had stem cell transplants, and up to 50,000 new transplants are done each year. 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 makes stem cells so important?
Stem cells make the 3 main types of blood cells: red blood cells, white blood cells, and platelets.
We need all of these types of blood cells to keep us alive. And in order for these blood cells to do their jobs, you need to have enough of each type in your blood.
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 blood test 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.
Another type of white blood cell the stem cells make are called lymphocytes. These are immune cells that can make antibodies and help fight infections, and includes the highly specialized T-lymphocytes (T cells), B-lymphocytes (B cells), and natural killer (NK) cells. Certain of these lymphocytes are responsible for the body’s ability to recognize its own cells and reject cells that are transplanted from someone else. This can play out differently after stem cells are transplanted from another donor. Then, it becomes the donor immune cells that decide what is “self” and what isn’t. The donor immune cells can attack the recipient’s body (see “Graft-versus-host disease” in the section called “Problems that may come up shortly after transplant.”)
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. This can be dangerous if bleeding occurs in the brain, or if blood begins to leak from the intestine or stomach.
More information on blood counts and what the numbers on your test results mean is available in our document called Understanding Your Lab Test Results which you can read on our Web site, www.cancer.org, or get by calling 1-800-227-2345.
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