What is a bone marrow transplant, and how does it work?

Bone marrow is the soft, spongy tissue inside your bones. It holds blood-forming stem cells, which can grow into every kind of blood cell.

Those cells become red blood cells, which carry oxygen; white blood cells, which help fight infection; and platelets, which help blood clot and stop bleeding. A transplant is used when the marrow is not working properly or has been destroyed by treatment.

You may hear several names for the same treatment: bone marrow transplant, blood stem cell transplant or hematopoietic (blood-forming) stem cell transplant. The name often reflects where the cells were collected. The US National Cancer Institute says most stem cells used in transplants come from the bloodstream rather than from the marrow itself.

A transplant can replace marrow that is diseased or has stopped working. It is used most often for cancers that affect blood cells, such as leukemia, lymphoma, multiple myeloma and myelodysplastic syndromes. It is also used for other blood disorders, including aplastic anemia, sickle cell disease and thalassemia.

For cancer, the transplant usually works by making very strong treatment possible. High doses of chemotherapy or radiation can destroy cancer cells, but they also destroy the marrow. The transplant restores the body’s ability to make new blood cells afterward.

Donor cells can add another benefit for some cancers, especially leukemia. White blood cells from the donor may attack cancer cells that are left after treatment. This is called the graft-versus-leukemia or graft-versus-tumor effect. The graft is simply the transplanted cells.

In an autologous transplant, the person’s own stem cells are collected, frozen and given back after strong treatment. No donor is needed. There is a small risk that cancer cells could be collected and returned along with the healthy cells.

In an allogeneic transplant, the cells come from someone else. The donor may be a brother or sister, a parent or child who is a half match, an unrelated volunteer on a donor registry, or donated umbilical cord blood. To find the best match they can, doctors compare HLA, the tissue-type markers found on most cells in the body.

Which kind a person has depends on the illness, their age and health, and whether a suitable donor is available. The care team weighs the risks and benefits of each option.

First, the transplant team checks whether the body is ready for the treatment. This usually means a physical exam, blood tests and heart and lung checks. Most people also get a central line: a soft tube placed in a large vein, usually in the chest, for medicines, blood draws and the new cells.

If donor cells are needed, the team tests the patient’s HLA type. They often test brothers and sisters and search donor registries at the same time, and the search can go on while other treatment continues. Patients do not have to find their own donor.

Most transplants use stem cells collected from the bloodstream. A machine draws blood, keeps the stem cells and returns the rest. Some donors give marrow instead, taken from the back of the hip bones under anesthesia. For an autologous transplant, the person’s own cells are collected and frozen ahead of time.

Next comes conditioning: chemotherapy, sometimes with radiation. It makes space in the marrow for the new cells and weakens or destroys the diseased cells. It also suppresses the immune system so the body does not reject donor cells. Some people get lower doses, sometimes called a mini transplant, which can make a transplant possible for older people and those with other health problems.

Transplant day is often called “day zero.” The cells drip into the central line from a bag, much like a blood transfusion. It is not surgery: the person stays awake, and it takes from under an hour to a few hours.

The new cells travel through the blood to the marrow and begin making red cells, white cells and platelets. This is called engraftment. Anthony Nolan, a UK stem cell charity, says it normally takes around two to three weeks but can take longer. Until then, blood counts are low, so infection and bleeding are real risks and transfusions are common.

People stay in the hospital, or visit the transplant center daily, until their blood counts start returning to normal. The American Cancer Society says this usually takes about 2 to 6 weeks. Many months of recovery follow, near the transplant center and then at home. NMDP, the US donor registry, says most transplant centers require a caregiver.

The immune system takes longest to rebuild. Even after blood counts are back to normal, the US National Cancer Institute says full immune recovery takes several months after an autologous transplant and 1 to 2 years after a donor transplant.

Infection is a real danger early on, because the body has few white blood cells at first and the immune system stays weak for months. Infections can be very serious. Medicines to prevent bacterial, fungal and viral infections are often given early on.

Graft-versus-host disease (GVHD) happens only after a donor transplant. Donor white blood cells see the patient’s body as foreign and attack it, which can damage the skin, liver, intestines and other organs. A closer match lowers the risk, and GVHD can be treated with steroids or other medicines that calm the immune system.

Graft failure means the new cells do not settle in the marrow and grow as they should, which can lead to serious bleeding or infection. The treatment can also affect the lungs, liver, kidneys and heart. Later problems can include infertility, cataracts and new cancers.

A transplant is not a guaranteed cure. It may cure an illness completely or only partly, and sometimes cancer comes back months or years later. Complications or a failed transplant can lead to death, which is why the care team weighs each person’s risks and benefits carefully.