When a patient receives bone marrow transplant what vital process is being restored?

Hematopoiesis

Hematopoiesis is the process by which new blood cells are produced in the bone marrow. When a patient receives a bone marrow transplant, the donor's bone marrow cells are infused into the patient's bloodstream, where they travel to the bone marrow and begin to produce new blood cells. This process can take several weeks to months, but eventually the patient's bone marrow will be fully functional and will be able to produce all of the different types of blood cells that the body needs.