The process of collecting (harvesting) stem cells for transplant depends on the source of the stem cells. Stem cells can be collected from bone marrow, circulating (peripheral) blood or umbilical cord blood.
Stem cells are 10–100 times more concentrated in bone marrow than in peripheral blood. The hip (pelvic) bone contains the largest amount of active marrow in the body and large numbers of stem cells. Harvesting stem cells from the bone marrow is usually done in the operating room.
Few risks are associated with donating bone marrow. The body usually replaces these cells within a few weeks. The doctor may suggest iron supplements until blood cell counts increase.
Sometimes blood is collected from the donor a few weeks before the bone marrow harvest. The blood is stored and given back to the donor after the stem cells are removed. This blood transfusion helps prevent anemia.
Harvesting stem cells from peripheral (circulating) blood is done on an outpatient basis. No anesthetic is needed.
Preparation before donation may be slightly different for allogeneic and autologous stem cell transplants.
The harvesting process takes 3–4 hours. Depending on the amount of stem cells collected, this process may be repeated daily for 1–5 days to collect enough stem cells for a transplant. The healthcare team checks blood cell counts daily during the donation process to make sure that levels do not fall too much below normal. Low blood cell counts can cause problems such as anemia.
Occasionally donors may have side effects during the collection process. Some people experience chills and light-headedness. Donors may also have numbness around the lips and in the fingers and toes or cramping of the hands. These side effects are due to low blood calcium levels, which is caused by the blood-thinning agent used during harvesting. They go away quickly when treated with calcium supplements.
Blood is collected from the umbilical cord shortly after a baby is born. The volume of stem cells collected per donation is quite small, so these cells are usually used for children or small adults.
Blood cells have a special antigen on their surface called CD34+. This antigen is important in stem cell transplant. The healthcare team can calculate the number of stem cells collected during harvesting using a method called the CD34+ assay. The number of stem cells needed for a transplant has not been formally determined, but it is based on the recipient’s weight.
Different transplant centres may have different standards for the number of stem cells needed. The range is 1–5 million CD34+ cells per kilogram of body weight. Less than 2 million CD34+ cells per kilogram of body weight can slow down engraftment.
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