All of the blood cells in our bodies start out as young (immature) cells called hematopoietic (blood-forming) stem cells. Even though they are often called "stem cells" these cells are not the same as the embryonic stem cells studied in therapeutic 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 mature they leave the bone marrow and enter the bloodstream. 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, and 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 measurement 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 a condition called 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 neutrophil count drops below 1,000 per cubic millimeter (1,000/mm3), a condition called neutropenia, 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. Lymphocytes develop from the earliest stage of stem cells.

Platelets (thrombocytes)

Platelets are fragments 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. People with this condition may bruise more easily, bleed longer, and have nose bleeds 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-ACS-2345.

Revised: 04/21/08