This page will try to answer that question.
Bone marrow contains special cells called 'stem cells'.
These cells are responsible for producing all the blood cells your body needs for your entire life. This means over 50 billion white blood cells every day in the average person. In a marrow transplant the 'broken' diseased stem cells are destroyed by chemotherapy and radiation and replaced with cells from a healthy donor. However, not any donor will do and it is critical to find a donor with the same tissue type. The problem is that there are millions and millions of different tissue types and although more than 2 million people have registered as marrow donors many patients cannot find a matched donor.