Allogeneic stem cell transplantation involves transferring the stem cells from a healthy person (the donor) to the patient’s body after high-intensity chemotherapy or radiation. The donated stem cells can come from either a related or an unrelated donor.

Before an allogeneic stem cell transplantation, the patient receives a conditioning regimen of chemotherapy and, sometimes, radiation therapy. This conditioning treatment is given to destroy any remaining cancer cells in the body. This helps weaken the patient’s immune system to help keep the body from rejecting the donated cells after the transplant. It also allows the donor cells to move through the bloodstream to the bone marrow, where the donor cells will begin to grow and produce new blood cells, including red blood cells, platelets and white blood cells. This process is called “engraftment.”

When a transplant is successful, the donor stem cells can replace stem cells in the bone marrow. It may also provide the only long-term cure of the patient’s disease. One of the benefits of allogeneic stem cell transplantation is that after the donated cells engraft in the patient, they create a new immune system. The donated cells produce white blood cells that attack any remaining cancer cells in the patient’s body. This is called the “graft-versus-tumor effect.” and it may be even more important than the very intensive conditioning regimen that is administered to destroy the cancer cells. This benefit can only occur in allogeneic stem cell transplantation.

One complication of allogeneic transplantation is that the patient’s body—despite the treatment to suppress the immune system—may reject the donated stem cells before they are able to engraft in the bone marrow. The patient’s immune cells may see the donor’s cells as foreign and destroy them.

Another complication of allogeneic transplantation is that the immune cells from the donor (the graft) may attack healthy cells in the patient’s body (host). This is called “graft-versus-host-disease" (GVHD). The parts of the body that are most commonly damaged by GVHD are the skin, intestines, liver, muscles, joints and eyes. Graft-versus-host disease can be mild, moderate or severe. There are treatments for GVHD, but in some patients, GVHD does not respond to treatment and can be fatal.

Allogeneic stem cell transplants for patients who are older or have overall poor health are relatively uncommon. This is because the pre-transplant conditioning therapy is generally not well tolerated by such patients, especially those with poorly functioning internal organs. However, reduced intensity allogeneic stem cell transplants may be an appropriate treatment for some older or sicker patients.