Bone marrow is the spongy tissue inside some of your bones, such as your hip and thigh bones. It contains immature cells, called stem cells. The stem cells can develop into the red blood cells that carry oxygen through your body, the white blood cells that fight infections, and the platelets help with blood clotting. If there is a problem with your bone marrow, a transplant can give you healthy new marrow. You could need a transplant because of a disease, such as bone marrow diseases or cancers like leukemia or lymphoma . Or you might need one if a strong cancer treatment kills your healthy blood cells. People with cancer sometimes donate bone marrow before treatment to be transplanted later. But often the new marrow comes from a donor, either a close family member or someone unrelated.