When the immune response is severely depressed-as the result of inherited defects, cancer therapy, or AIDS-one possible remedy is a transfer of healthy bone marrow. Bone marrow transplants are also used to treat patients with cancers of the blood, the blood-forming organs, and the lymphoid system-the leukemias and lymphomas.

Once in the circulation, transplanted bone marrow cells travel to the bones where the immature cells grow into functioning B and T cells. Like other transplanted tissue, however, bone marrow from a donor must carry self markers that closely match those of the person intended to receive it. This match is essential not only to prevent the transplant from being rejected, but also to fend off a life-threatening situation known as graft-versus-host disease. In graft-versus-host disease, mature T cells from the donor attack and destroy the tissues of the recipient.

To prevent graft-versus-host disease, scientists have developed techniques to "cleanse" the donor marrow of potentially dangerous mature cells. These include chemicals and, more recently, a monoclonal antibody (OKT3) that specifically recognizes and eliminates mature T cells.

For cancer patients who face immunosuppressive therapy but who have no readily matched donor, doctors have used "autologous" transplants: the person's bone marrow is removed, frozen, and stored until therapy is complete; then the cells are thawed and reinfused.