To kill cancer cells and suppress the immune system so it won’t reject the donor cells.

At its simplest, a bone marrow transplant replaces a damaged or diseased immune system with healthy stem cells. These cells are the "architects" of your blood, responsible for creating red cells (oxygen), white cells (immunity), and platelets (clotting). The Two Primary Types

Before the new cells enter, the old system must be cleared. Patients undergo "conditioning"—intense chemotherapy or radiation.