from a variety of sources including bone marrow, peripheral blood, and umbilical cord blood collected from the placentas of recently delivered infants. Because of the potential of HPCs to reconstitute bone marrow and peripheral blood, their use for the treatment of patients with bone marrow damage from either chemotherapy or underlying hematological failure has been under investigation for several decades. Transplantation of HPCs from healthy individuals could also reconstitute bone marrow or blood in individuals with a variety of blood-related disorders (human-to-human transfer is called “allogeneic transplantation”).

Early research specifically into cord blood transplantation was based on the hypothesis that the immune cells in cord blood may be less mature than those in adult bone marrow or peripheral blood. Consequently, the risk of graft-versus-host disease (GVHD)3 after a cord blood transplant might be less than that after a bone marrow transplant or peripheral blood transplant. Other advantages of cord blood over bone marrow include its ready availability, its low potential for infectious disease transmission, and the minimal risk at the time of collection. This opens up the possibility of HPC transplantation to patients who either could not find a match within the bone marrow donor pool or were too ill to be able to wait for the process of searching and harvesting of bone marrow from adult donors.

Since the first transplants in the late 1980s and early 1990s, cord blood has been shown to be a suitable alternative to adult bone marrow or peripheral blood as a source of HPCs for the treatment of leukemia, lymphoma, aplastic anemia, and inherited disorders of immunity and metabolism. Whether cord blood is as good as or superior to adult graft sources in all of these situations is as yet unknown. Factors already shown to influence the outcome of cord blood transplantation include the numbers of cells in the cord blood, the size of the recipient, and the degree of human leukocyte antigen (HLA) match4 between the donor and the recipient. The committee holds the view that cord blood and marrow are complementary sources of HPC, each having specific advantages and disadvantages, and that the choice between the two should be made on a case-by-case basis, depending on the status of the patient.

3  

GVHD occurs when donor cells attack the recipient’s normal tissues after transplant and can lead to organ damage.

4  

HLA stands for human leukocyte antigen, the major histocompatibility complex in humans. The closer the donor’s and recipient’s HLA antigens match, the less likely it is that the T cells (immune system cells) of the donated marrow will react against the patient’s body. Within a family, siblings have one-in-four chance of being HLA-identical. Outside the family, the situation is very different. HLA antigens are highly polymorphic, with hundreds of different HLA antigens found in the human population (there are roughly 750,000 possible combinations of three HLA antigens alone). To find an unrelated HLA-matched donor requires searching very large numbers of people (Beatty et al., 1988).



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