Allogeneic transplantation from donors other than HLA-identical siblings

Most patients who could benefit from allogeneic marrow transplant do not have a histocompatible sibling donor and thus, potentially curative therapy often is possible only through the use of an alternative donor. The current scope of alternative donors ensures access to allogeneic transplantation for almost every patient in need. Sources of alternative donors include unrelated sources, such as phenotypically HLA-similar adult volunteers or cryopreserved cord blood units, or genetically related haploidentical family members. Clinical and laboratory considerations and patient outcomes using these alternative donors are reviewed in this article. In general, transplant procedures using alternative donors are more difficult to perform and and carry a greater risk of complication as a result of immunologic barriers imposed by major and minor HLA differences. Ongoing developments aimed at improving engraftment, the control of acute and chronic graft-versus-host disease, and enhanced immune reconstitution may eventually lead to equivalent outcomes compared with the use of a matched sibling.