hortly after 5 a.m., Teri Majewski marches purposefully into Rush-Presbyterian-St. Luke's Medical Center in Chicago. Strongly built and fit looking, the 34-year-old mother of two young children seems too healthy to be in a hospital. But she checks in at the day-surgery department and is summoned to an examining room, from which she emerges a few minutes later in a baggy blue hospital gown and the inevitable plastic bracelet. Suddenly, she looks vulnerable. This morning Majewski is scheduled to undergo a bone-marrow harvest, in which doctors will remove about a quart of her marrow to be transplanted into a young patient dying of leukemia. She has never met the patient, who lives in Europe. They do not even know each other's names. They have been brought together by a computer search, by the quirk of a few shared genetic traits and, above all, by Majewski's kindness and courage, her willingness to endure a painful procedure in the hope of saving the life of a perfect stranger.

Bone-marrow transplantation is often the last hope for people with devastating diseases: leukemia and other cancers, and certain genetic disorders of the blood, immune system or metabolism. Cure rates range from 20% to 80%, depending on the disease, its stage and the degree of compatibility between the donor's marrow and the recipient.

For the recipient, the process begins with massive doses of chemotherapy or radiation, or both, to wipe out the disease. But that treatment kills the patient's bone-marrow cells as well. Without this spongy tissue at the core of many larger bones, a person cannot live. Marrow contains the precious stem cells that produce all the body's 30,000 trillion red blood cells, many of its infection-fighting white cells and the platelets that are essential for clotting.