Bone marrow is the tissue inside bones that makes red blood cells, white blood cells, and platelets.
Each of these plays a vital role in keeping the body healthy and free of disease. If you lack red cells, which carry oxygen, you get very fatigued. Without white cells, your body can't protect itself from infections. You need platelets for the clotting of blood. The technique involves removing the bone marrow from a compatible donor, and then giving it to the patient.