Who is the best match for a stem cell transplant?

Most often a brother or sister. That is the National Cancer Institute's answer for an allogeneic transplant, which means one using stem cells from another person.

What is being matched

Donor cells are tested for their HLAs, short for human leukocyte antigens. HLAs are sets of proteins, or markers, found on most cells in the body, and each person has a different set.

NCI's rule is simple. The more HLAs the patient and the donor have in common, the better the chance the body will accept the donor's cells.

NCI does not publish the odds that a given sibling will match, and no percentage appears on its page. What it does say is that a donor can also be someone unrelated, if the cells are a close enough match. So a transplant is not off the table when no relative fits.

Why closeness matters so much

Matching is not paperwork. It decides how likely two serious problems are.

The first is rejection. NCI says the cells must match closely enough that the immune system does not see them as foreign and destroy them.

The second is graft-versus-host disease, or GVHD. That happens when white blood cells from the donor, called the graft, see cells in the patient's body, the host, as foreign and attack them. NCI says it can damage skin, liver, intestines, and many other organs.

GVHD comes in two forms. Acute GVHD occurs within the first 3 months after transplant. Chronic GVHD occurs at 3 months or later.

NCI names three ways of lowering the risk. The closer the match, the less likely GVHD is. Drugs can suppress the immune system. And donated cells can be treated to remove the T cells that cause it, a process called T-cell depletion.

When a donor is not needed at all

Not every transplant involves matching. NCI describes three kinds.

Autologous transplants use the patient's own stem cells. The cells match by definition, but there is a small risk that cancer cells are transplanted along with them.

Allogeneic transplants use cells from someone else, related or not.

Syngeneic transplants use cells from an identical twin.

There is also a mini-transplant, a type of allogeneic transplant using lower doses of treatment beforehand. It does not kill all of the patient's blood-forming stem cells, but it still kills some cancer cells.

Cells themselves can come from three places: the bloodstream, in a peripheral blood stem cell transplant; bone marrow, in a bone marrow transplant; or umbilical cord blood collected when a baby is born.

The trade-off nobody expects

Here is the part that makes matching more complicated than "closer is better."

NCI describes a graft-versus-tumor effect, in which donor white blood cells attack any cancer cells left in the body. It says this effect improves the chances that the transplant succeeds.

So the same immune difference that causes GVHD also fights the cancer. That is why transplant teams weigh donor options rather than simply picking the closest one, and why an autologous transplant, which has no matching problem, also has no graft-versus-tumor effect.

What else goes into the decision

NCI lists what a doctor weighs: the type of cancer, how advanced it is, whether the patient's own cells can be used, whether matching donor cells are available, whether other treatments are likely to work, whether the patient can tolerate high doses of chemotherapy, other serious health problems, and past treatments.

Practical planning belongs in that conversation too. NCI notes that a patient with no problems can go home 100 days after receiving donor cells, and will need close follow-up from a doctor experienced in transplant care. Our guide to stem cell transplant covers the process, and preparing for a stem cell transplant covers what comes first.

NCI's page was last updated on October 5, 2023.

Sources

• https://www.cancer.gov/about-cancer/treatment/types/stem-cell-transplant