If you are planning to donate blood and bone marrow, you've agreed to allow doctors to draw blood stem cells from your blood or bone marrow for transplantation. Blood stem cells are the cells that make all of the body's blood cells. They form and mature in the bone marrow, and are then released into the bloodstream. Although they're called "stem cells," these cells aren't the same as the embryonic stem cells studied in therapeutic cloning and other types of research.
In the past, surgery to draw marrow from the bone was the only way to collect blood stem cells. Today, however, it's more common to collect blood stem cells directly from the blood. This is called peripheral blood stem cell (PBSC) donation.
Blood stem cells can also be collected from umbilical cord blood at birth. However, only a small amount of blood can be retrieved from the umbilical cord, so this type of transplant is generally reserved for children and small adults.