A life-saving transplant can depend on finding one person.

Blood begins here.

Bone marrow is the soft tissue inside bones where blood cells are made. It continually replaces cells that have reached the end of their lives.

In a healthy adult, production of red cells, platelets and neutrophils adds up to more than 300 billion cells each day.

A small population keeps the supply going.

Blood-forming stem cells can renew themselves and produce the cells that develop into the different parts of our blood.

In a blood stem cell transplant, these are the cells that can establish a new source of blood production.

This story is about blood-forming stem cells, which can be collected from blood, marrow or donated cord blood.

Different cells do different jobs.

Blood-cell development is often taught as two broad families: myeloid and lymphoid. This helps explain the names of different leukemias.

Three essential jobs

The branching illustration simplifies a complex, continuous process. Acute leukemia develops quickly; chronic leukemia often develops more slowly.

Leukemia changes how blood cells develop.

Leukemia is a group of cancers of blood-forming tissue. Abnormal blood cells build up and can interfere with healthy blood production.

In acute leukemias such as AML and ALL, many of these cells are immature cells called blasts. They do not develop or function normally. Chronic leukemias behave differently and may contain more mature-looking cells.

The animation illustrates acute leukemia. It does not show every type of leukemia or any individual person’s disease.

Healthy blood production can fall.

As leukemia cells accumulate in the marrow, fewer healthy red cells, platelets and infection-fighting white cells may be produced.

That helps explain why a disease of the marrow can affect the whole body. Leukemia cells can also collect in tissues outside the marrow.

Symptoms need a medical assessment.

Acute leukemia can cause tiredness, fever, infections, easy bruising or bleeding. These symptoms can also have many other causes.

How low blood counts can affect the body

• Too few red cells can cause anemia, with tiredness, paleness or breathlessness.
• Too few platelets can make bruising or bleeding more likely.
• Too few functioning white cells can make it harder to fight infection.

Symptoms alone cannot diagnose leukemia. Persistent or concerning symptoms should be assessed by a healthcare professional. People already receiving cancer treatment should follow their team’s urgent-contact instructions for fever or other new symptoms.

Blood tests start the investigation.

A complete blood count and examination of blood cells can reveal changes that need investigating. A diagnosis usually combines these findings with other tests, often including a bone marrow sample.

Tests of cell markers, chromosomes and genes help identify the leukemia subtype and guide treatment. An abnormal blood count on its own is not a diagnosis.

A marrow sample taken to diagnose a patient is different from the collection procedure used when a donor gives marrow.

Four common names describe different diseases.

Acute or chronic describes how the disease tends to develop. Myeloid or lymphoid describes the blood-cell family involved.

The four main types

• AML — acute myeloid leukemia. Usually progresses quickly and requires prompt treatment.
• ALL — acute lymphoblastic leukemia. Also progresses quickly; treatment differs between children and adults.
• CML — chronic myeloid leukemia. Often managed with medicines called tyrosine kinase inhibitors.
• CLL — chronic lymphocytic leukemia. Some people are monitored without treatment until it is needed.

Each type has further subtypes. A leukemia name alone cannot predict someone’s treatment or outcome.

Treatment follows the biology of the disease.

Treatment may include chemotherapy, targeted medicines, immunotherapy or a transplant. The choices depend on the leukemia subtype, test results, response to treatment, age and overall health.

AML treatment commonly includes an initial phase to reach remission and further treatment to reduce relapse risk. ALL treatment usually has several phases, including treatment to protect the brain and spinal cord; many regimens also include maintenance.

Side effects vary. Supportive care can include medicines for symptoms and infections, and red-cell or platelet transfusions. Transfusions support the patient; they are different from a blood stem cell transplant.

Remission is an important milestone.

Remission means the signs of leukemia have reduced or disappeared according to clinical tests. It is not the same as a guarantee that the disease will never return.

Sensitive tests for measurable residual disease can detect very small amounts of leukemia. Those results, the disease’s genetics and the response to treatment help a team weigh the next step.

When a donor transplant may be needed, HLA testing and a donor search can begin early, alongside treatment. The search does not have to wait for a relapse.

A donor transplant is one possible treatment.

Many people with leukemia are treated without a donor transplant. Some people with AML or ALL may benefit from one, including selected patients with a higher risk of relapse or disease that has returned.

An allogeneic transplant uses another person’s blood-forming cells. An autologous transplant returns the patient’s own cells. The donor search in this story concerns allogeneic transplantation.

A transplant can offer a chance of long-term disease control or cure, but it also carries serious risks. Patients and their teams weigh those risks against other options; there is no single rule for everyone with leukemia.

Treatment prepares the body for new cells.

Before a transplant, a patient receives a preparation regimen called conditioning. It uses chemotherapy, sometimes with radiation, to treat the disease and suppress the immune response to donated cells.

Conditioning intensity varies. Some regimens use lower doses of chemotherapy or radiation. The team selects the approach for the treatment plan and the patient’s health.

Low blood counts and infection risk require close monitoring and supportive care. The regimen and its risks are explained by the transplant team.

The new cells enter through a vein.

The cell infusion is usually given through a central line, much like a blood transfusion. The patient is monitored throughout. This part of the transplant does not involve an operation to place cells inside the bones.

Often called day zero, it begins the next phase of treatment. The donated cells travel through the bloodstream to the marrow.

Engraftment means new blood production begins.

When the donated cells establish themselves and start making new blood cells, this is called engraftment. The team monitors blood counts and other tests to check how recovery is progressing.

It often takes weeks. During this time, patients may need red-cell and platelet transfusions, infection prevention and other support.

Engraftment is a clinical milestone, not one isolated rise in a blood count. It does not mean the immune system has fully recovered or that the leukemia is cured.

Recovery continues after leaving hospital.

Recovery can take many months. The immune system may recover more slowly than blood counts, and the team sets a plan for medicines, infection precautions, vaccinations and follow-up.

Caregivers can help with appointments and daily life. Returning to work, school or exercise is individual; there is no fixed timetable that every patient should meet.

The treating team can help with physical recovery, emotional support and practical concerns such as travel, housing and finances.

Donor immune cells can help — and cause harm.

Donor immune cells may recognize and attack remaining leukemia cells. This graft-versus-leukemia effect is one reason an allogeneic transplant can be effective.

They can also attack healthy tissues in the patient. This is graft-versus-host disease, or GVHD. It can be serious and sometimes life-threatening. Teams use donor selection and preventive treatment to reduce its risk.

GVHD can affect different organs and may persist long term. Its likelihood and severity depend on several factors, including the prevention regimen.

Long-term follow-up matters.

After a transplant, teams continue checking for relapse, GVHD and late effects of treatment. These can include fertility problems or effects on organs, as well as a risk of later cancers.

These are possible complications, not a prediction for any individual. Follow-up helps clinicians identify and manage them. A successful transplant can make a lasting difference, but no donor or treatment can promise an outcome.

The next chapters explain how donor matching fits into that carefully planned care.

A match is more than a blood type.

Human leukocyte antigens, or HLA, are proteins found on most cells. They display protein fragments that help immune cells recognize what is happening inside the body.

For a donor transplant, laboratories compare inherited HLA types. This is different from matching red-blood-cell groups such as A, B or O.

HLA comes in many inherited combinations.

HLA genes have many versions. A patient’s combination may be common in a registry, or it may be difficult to find.

Similar ancestry can make a shared HLA type more likely, but appearance, nationality or an ethnic label cannot tell whether two people match. Laboratory testing is essential.

What does 8 out of 8 mean?

One common unrelated-donor comparison checks two copies each of HLA-A, HLA-B, HLA-C and HLA-DRB1. An 8/8 match means those eight copies match at the level tested.

Teams also assess other HLA genes and factors such as donor age, availability and the treatment used to prevent graft-versus-host disease. A full match at the markers compared does not mean two people are genetically identical.

HLA matters, but the highest match count alone does not always identify the best donor. Current approaches can use partially matched donors for some patients.

Cell-surface marker

HLA proteins are found on most cells and form part of the body's biological identification system.

Peptide-binding groove

The groove displays small protein fragments that immune cells can inspect.

Immune recognition

Immune cells use HLA-related signals to help distinguish the body's cells from cells that appear foreign.

Inherited code

A person inherits half of their HLA information from each biological parent.

Family members may share an HLA pattern.

HLA genes are close together on a chromosome and are usually inherited as a group, called a haplotype. A person inherits one group from each biological parent.

Parents and children share an inherited half-set. That can make them potential donors for a haploidentical, or half-matched, transplant.

A team confirms HLA types by testing. It also checks a potential donor’s health and suitability.

Each full sibling has a one-in-four chance.

For siblings who share both biological parents, each sibling has a 25% chance of inheriting the same two HLA groups as the patient.

The calculator shows the chance that at least one full sibling matches. It does not predict whether a matching sibling is medically able to donate.

Family and registry searches can run together.

A transplant team may test relatives and search for unrelated donors or cord blood at the same time. It does not always wait for one route to fail before starting another.

A family relationship does not guarantee a suitable donor. A relative who cannot donate has not failed the patient.

Any donation requires assessment, information and voluntary consent.

The search can cross borders.

Registries and cord blood banks cooperate internationally so transplant teams can look beyond a patient’s own country. The World Marrow Donor Association supports this network.

A donor in one country may help a patient in another. People join through an eligible registry serving the country where they live.

A registry lists potential donors; a cord blood bank stores donated units. Neither a listed person nor a stored unit is automatically suitable or available for a particular patient.

Representation affects who can be found.

HLA is inherited, and its patterns vary between populations. People with similar ancestry are more likely to share a type, although matches can cross ethnic backgrounds.

Registries do not represent every community equally. Some inherited combinations, including those found in people of mixed ancestry, can be harder to find.

The illustration groups similar HLA patterns. Its colors and positions do not represent races, countries, actual donors or anyone’s personal odds.

More donors and more treatment options both help.

Recruiting people from many backgrounds expands the range of HLA types a team can search. Keeping registered donors informed and reachable matters too.

Research has also improved the use of partially matched donors. A search for a suitable graft can include these options early; finding no full match does not necessarily mean there is no transplant option.

Matching is one part of access. Timing, donor availability, specialist care and access to treatment also matter.

A possible match still needs to be checked.

When a possible donor is identified, the registry confirms contact details, willingness, health and the HLA match. More than one candidate may be checked.

If you are already registered

• Keep your contact details current with your registry.
• Tell the registry about health or availability changes when it asks.
• Respond promptly if contacted, even if you are unsure you can donate.

A medical deferral or a decision not to donate can change a search. Registries discuss the commitment and support available so donors can make an informed choice.

Start with the registry where you live.

Age limits, health criteria and the joining process differ by registry. A registration check is followed by a more detailed assessment if someone is selected to donate.

You do not need to decide your own eligibility from a list on this page. Read the official registry’s current guidance and ask its staff about any uncertainty.

JBF helps people find independent registries. It does not decide eligibility, collect swabs or perform HLA testing.

A full match is valuable. It is not always the only path.

Depending on the patient and treatment plan, a team may consider a matched unrelated donor, a mismatched unrelated donor, a haploidentical relative or a cord blood unit.

The comparison below is a simplified example, not a tool for choosing a donor.

Blood-forming cells have three main sources.

Donated cells can come from the bloodstream after preparation medicines, from the marrow of the pelvic bones under anesthesia, or from donated umbilical cord blood stored in a bank.

The source affects the number and type of cells available, the recovery of blood counts and some transplant risks. The team chooses it as part of the treatment plan.

Cord blood can offer another option.

Blood remaining in the umbilical cord and placenta after birth contains blood-forming cells. With consent, it can be collected, tested and stored for a future transplant.

Cord blood can sometimes be used with a less exact HLA match than an adult-donor graft. HLA compatibility still matters.

Every source has practical limits.

For cord blood, a team must consider the cell dose and quality of the stored unit as well as matching. For an adult donor, health, willingness and availability must be confirmed.

Public cord blood donation is offered only through participating programs and hospitals. Families can ask their maternity team about local options before delivery.

Public donation makes a unit available to suitable patients. It is different from paying for private storage for a family; neither route guarantees that the unit will be used.

Matching works together with the treatment plan.

HLA differences can affect immune reactions, graft failure and graft-versus-host disease. Their importance depends in part on which genes differ and how complications are prevented.

Teams weigh the match alongside donor age, donor-specific antibodies, cell source, availability and the patient’s needs.

There is no universal rule that each additional matching marker guarantees a better outcome.

New approaches have expanded donor options.

Medicines such as post-transplant cyclophosphamide have improved outcomes with mismatched unrelated and haploidentical donors. NMDP’s 2025 guidelines advise considering these options early when appropriate.

That progress makes donor selection more flexible. It also makes the full clinical picture more important than a simple match score.

Results depend on the patient, disease and regimen. These advances do not guarantee a suitable available donor, access to transplant or a successful outcome everywhere.

Full match

Donor and patient match at every marker compared.

Partial match

This example differs at one of eight markers. Mismatched unrelated donors can differ at more than one; suitability depends on the treatment plan.

Haploidentical match

A relative shares one inherited HLA half-set. The diagram simplifies this to four of eight markers; actual matching can overlap further.