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Hematopoietic progenitors cells (HPC) were first demonstrated in human peripheral blood (PB) in 1971.1 In normal individuals small amounts of peripheral blood stem cells (PBSC) are present in the PB during steady state hematopoiesis and many aphereses are required to obtain adequate PBSC for transplant. In the mid 1980s, autologous PBSC transplantations (PBSCT) were performed successfully as an alternative to bone marrow transplantation (BMT) in patients with malignant lymphoma, breast cancer, and acute leukemia.2-4 These studies showed that hematopoietic engraftment can be achieved with stem cells collected from the circulating blood rather than from the bone marrow (BM) and hematopoietic reconstitution using stem cells collected during steady-state was similar to BM. However, while most patients experienced a complete and stable hematopoietic reconstitution, some patients had slow platelet recovery or experienced a subsequent fall in PB count with nonmobilized PBSCT. This raises some concerns about the ability of circulating stem cells collected during steady state to sustain life-long hematopoiesis. Some investigators added PBSC to autologous BM grafts in an attempt to shorten the duration of neutropenia and enhance hematopoi-etic recovery. However, only the addition of PBSC collected after prior myelosuppressive therapy,5 not during the steady state,6 were found to accelerate hematopoi-etic recovery and prompt engraftment.

Blood progenitor cell mobilization in humans was initially noted during recovery from myelosuppressive chemotherapy.7 The advances in knowledge in stem cell biology, the availability of hematopoietic growth factors, the availability of large-scale, continuous-flow leukopheresis, and an improved technique for progenitor cells assay, have increased the use of mobilized PBSC for transplantation. PBSC have several advantages when compared to BM grafts, including the fact that it is an outpatient procedure and no general anesthesia is required. In addition, results from randomized studies have shown that the hematopoietic recovery following myeloablative therapy was much more rapid with mobilized PBSC autografts than with BM autografts followed by growth factor.8 This has lead to the widespread use of mobilized PBSC in all autologous transplant settings and in many allogeneic transplant settings.

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