All you need to know about bone marrow

Bone marrow is soft, gelatinous tissue that fills the medullary cavities, or the centers of bones. The two types of bone marrow are red bone marrow, known as myeloid tissue, and yellow bone marrow, known as fatty tissue.

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Both types of bone marrow are enriched with blood vessels and capillaries.

Bone marrow makes more than 220 billion new blood cells every day. Most blood cells in the body develop from cells in the bone marrow.

Bone marrow stem cells

Bone marrow contains two types of stem cells: mesenchymal and hematopoietic.

Red bone marrow consists of a delicate, highly vascular fibrous tissue containing hematopoietic stem cells. These are blood-forming stem cells.

Yellow bone marrow contains mesenchymal stem cells, or marrow stromal cells. These produce fat, cartilage, and bone.

Stem cells are immature cells that can turn into a number of different types of cells.

Hematopoietic stem cells in the bone marrow give rise to two main types of cells: myeloid and lymphoid lineages. These include monocytes, macrophages, neutrophils, basophils, eosinophils, erythrocytes, dendritic cells, and megakaryocytes, or platelets, as well as T cells, B cells, and natural killer (NK) cells.

The different types of hematopoietic stem cells vary in their regenerative capacity and potency. They can be multipotent, oligopotent, or unipotent, depending on how many types of cells they can create.

Pluripotent hematopoietic stem cells have renewal and differentiation properties. They can reproduce another cell identical to themselves, and they can generate one or more subsets of more mature cells.

The process of developing different blood cells from these pluripotent stem cells is known as hematopoiesis. It is these stem cells that are needed in bone marrow transplants.

Stem cells constantly divide and produce new cells. Some new cells remain as stem cells, while others go through a series of maturing stages, as precursor or blast cells, before becoming formed, or mature, blood cells. Stem cells rapidly multiply to make millions of blood cells each day.

Blood cells have a limited life span. This is around 120 days for red blood cells. The body is constantly replacing them. The production of healthy stem cells is vital.

The blood vessels act as a barrier to prevent immature blood cells from leaving bone marrow.

Only mature blood cells contain the membrane proteins required to attach to and pass through the blood vessel endothelium. Hematopoietic stem cells can cross the bone marrow barrier, however. Healthcare professionals may harvest these from peripheral, or circulating, blood.

The blood-forming stem cells in red bone marrow can multiply and mature into three significant types of blood cells, each with its own job:

• Red blood cells (erythrocytes): These transport oxygen around the body.
• White blood cells (leukocytes): These help fight infection and disease. White blood cells include lymphocytes, which make up the cornerstone of the immune system, and myeloid cells, which include granulocytes, neutrophils, monocytes, eosinophils, and basophils.
• Platelets (thrombocytes): These help with blood clotting after injury. Platelets are fragments of the cytoplasm of megakaryocytes, which are another type of bone marrow cell.

Once mature, these blood cells move from bone marrow into the bloodstream, where they perform important functions that keep the body alive and healthy.

Mesenchymal stem cells are present in the bone marrow cavity. They can differentiate into a number of stromal lineages, such as:

• chondrocytes (cartilage generation)
• osteoblasts (bone formation)
• osteoclasts
• adipocytes (adipose tissue)
• myocytes (muscle)
• macrophages
• endothelial cells
• fibroblasts

Red bone marrow

Red bone marrow produces all red blood cells and platelets and around 60–70% of lymphocytes in human adults. Other lymphocytes begin life in red bone marrow and become fully formed in the lymphatic tissues, including the thymus, spleen, and lymph nodes.

Together with the liver and spleen, red bone marrow also plays a role in getting rid of old red blood cells.

Yellow bone marrow

Yellow bone marrow mainly acts as a store for fats. It helps provide sustenance and maintain the correct environment for the bone to function. However, under particular conditions — such as with severe blood loss or during a fever — yellow bone marrow may revert to red bone marrow.

Yellow bone marrow tends to be located in the central cavities of long bones and is generally surrounded by a layer of red bone marrow with long trabeculae (beam-like structures) within a sponge-like reticular framework.

Bone marrow timeline

Before birth but toward the end of fetal development, bone marrow first develops in the clavicle. It becomes active about 3 weeks later. Bone marrow takes over from the liver as the major hematopoietic organ at 32–36 weeks’ gestation.

Bone marrow remains red until around the age of 7 years, as the need for new continuous blood formation is high. As the body ages, it gradually replaces the red bone marrow with yellow fat tissue. Adults have an average of about 2.6 kilograms (kg) (5.7 pounds) of bone marrow, about half of which is red.

In adults, the highest concentration of red bone marrow is in the bones of the vertebrae, hips (ilium), breastbone (sternum), ribs, and skull, as well as at the metaphyseal and epiphyseal ends of the long bones of the arm (humerus) and leg (femur and tibia).

All other cancellous, or spongy, bones and central cavities of the long bones are filled with yellow bone marrow.

Most red blood cells, platelets, and most white blood cells form in the red bone marrow. Yellow bone marrow produces fat, cartilage, and bone.

White blood cells survive from a few hours to a few days, platelets for about 10 days, and red blood cells for about 120 days. Bone marrow needs to replace these cells constantly, as each blood cell has a set life expectancy.

Certain conditions may trigger additional production of blood cells. This may happen when the oxygen content of body tissues is low, if there is loss of blood or anemia, or if the number of red blood cells decreases. If these things happen, the kidneys produce and release erythropoietin, which is a hormone that stimulates bone marrow to produce more red blood cells.

Bone marrow also produces and releases more white blood cells in response to infections and more platelets in response to bleeding. If a person experiences serious blood loss, yellow bone marrow can activate and transform into red bone marrow.

Healthy bone marrow is important for a range of systems and activities.

Circulatory system

The circulatory system touches every organ and system in the body. It involves a number of different cells with a variety of functions. Red blood cells transport oxygen to cells and tissues, platelets travel in the blood to help clotting after injury, and white blood cells travel to sites of infection or injury.

Hemoglobin

Hemoglobin is the protein in red blood cells that gives them their color. It collects oxygen in the lungs, transports it in the red blood cells, and releases oxygen to tissues such as the heart, muscles, and brain. Hemoglobin also removes carbon dioxide (CO₂), which is a waste product of respiration, and sends it back to the lungs for exhalation.

Iron

Iron is an important nutrient for human physiology. It combines with protein to make the hemoglobin in red blood cells and is essential for producing red blood cells (erythropoiesis). The body stores iron in the liver, spleen, and bone marrow. Most of the iron a person needs each day for making hemoglobin comes from the recycling of old red blood cells.

Red blood cells

The production of red blood cells is called erythropoiesis. It takes about 7 days for a committed stem cell to mature into a fully functional red blood cell. As red blood cells age, they become less active and more fragile.

White blood cells called macrophages remove aging red cells in a process known as phagocytosis. The contents of these cells are released into the blood. The iron released in this process travels either to bone marrow for the production of new red blood cells or to the liver or other tissues for storage.

Typically, the body replaces around 1% of its total red blood cell count every day. In a healthy person, this means that the body produces around 200 billion red blood cells each day.

White blood cells

Bone marrow produces many types of white blood cells. These are necessary for a healthy immune system. They prevent and fight infections.

The main types of white blood cells, or leukocytes, are as follows.

Lymphocytes

Lymphocytes are produced in bone marrow. They make natural antibodies to fight infection due to viruses that enter the body through the nose, mouth, or another mucous membrane or through cuts and grazes. Specific cells recognize the presence of invaders (antigens) that enter the body and send a signal to other cells to attack them.

The number of lymphocytes increases in response to these invasions. There are two major types of lymphocytes: B and T lymphocytes.

Monocytes

Monocytes are produced in bone marrow. Mature monocytes have a life expectancy in the blood of only 3–8 hours, but when they move into the tissues, they mature into larger cells called macrophages.

Macrophages can survive in the tissues for long periods of time, where they engulf and destroy bacteria, some fungi, dead cells, and other material that is foreign to the body.

Granulocytes

“Granulocytes” is the collective name given to three types of white blood cells: neutrophils, eosinophils, and basophils. The development of a granulocyte may take 2 weeks, but this time reduces when there is an increased threat, such as a bacterial infection.

Bone marrow stores a large reserve of mature granulocytes. For every granulocyte circulating in the blood, there may be 50–100 cells waiting in the bone marrow to be released into the bloodstream. As a result, half the granulocytes in the bloodstream can be available to actively fight an infection in the body within 7 hours of it detecting one.

Once a granulocyte has left the blood, it does not usually return. A granulocyte may survive in the tissues for up to 4–5 days, depending on the conditions, but it can only survive for a few hours in circulating blood.

Neutrophils

Neutrophils are the most common type of granulocyte. They can attack and destroy bacteria and viruses.

Eosinophils

Eosinophils are involved in the fight against many types of parasitic infections and against the larvae of parasitic worms and other organisms. They are also involved in some allergic reactions.

Basophils

Basophils are the least common of the white blood cells. They respond to various allergens that cause the release of histamines, heparin, and other substances.

Heparin is an anticoagulant. It prevents blood from clotting. Histamines are vasodilators that cause irritation and inflammation. Releasing these substances makes a pathogen more permeable and allows for white blood cells and proteins to enter the tissues to engage the pathogen.

The irritation and inflammation in tissues that allergens affect are parts of the reaction associated with hay fever, some forms of asthma, hives, and, in its most serious form, anaphylactic shock.

Platelets

Bone marrow produces platelets in a process known as thrombopoiesis. Platelets are necessary for blood to coagulate and for clots to form in order to stop bleeding.

Sudden blood loss triggers platelet activity at the site of an injury or wound. Here, the platelets clump together and combine with other substances to form fibrin. Fibrin has a thread-like structure and forms an external scab or clot.

Platelet deficiency causes the body to bruise and bleed more easily. Blood may not clot well at an open wound, and there may be a higher risk of internal bleeding if the platelet count is very low.

Lymphatic system

The lymphatic system consists of lymphatic organs such as bone marrow, the tonsils, the thymus, the spleen, and lymph nodes.

All lymphocytes develop in bone marrow from immature cells called stem cells. Lymphocytes that mature in the thymus gland (behind the breastbone) are called T cells. Those that mature in bone marrow or the lymphatic organs are called B cells.

Immune system

The immune system protects the body from disease. It kills unwanted microorganisms such as bacteria and viruses that may invade the body.

How does the immune system fight infection?

Small glands called lymph nodes are located throughout the body. Once lymphocytes are made in bone marrow, they travel to the lymph nodes. The lymphocytes can then travel between each node through lymphatic channels that meet at large drainage ducts that empty into a blood vessel. Lymphocytes enter the blood through these ducts.

Three major types of lymphocytes play an important part in the immune system: B lymphocytes, T lymphocytes, and NK cells.

B lymphocytes (B cells)

These cells originate from hematopoietic stem cells in bone marrow in mammals.

B cells express B cell receptors on their surface. These allow the cell to attach to an antigen on the surface of an invading microbe or another antigenic agent.

For this reason, B cells are known as antigen-presenting cells, as they alert other cells of the immune system to the presence of an invading microbe.

B cells also secrete antibodies that attach to the surface of infection-causing microbes. These antibodies are Y-shaped, and each one is akin to a specialized “lock” into which a matching antigen “key” fits. Because of this, each Y-shaped antibody reacts to a different microbe, triggering a larger immune system response to fight infection.

In some circumstances, B cells erroneously identify healthy cells as being antigens that require an immune system response. This is the mechanism behind the development of autoimmune conditions such as multiple sclerosis, scleroderma, and type 1 diabetes.

T lymphocytes (T cells)

These cells are so-called because they mature in the thymus, which is a small organ in the upper chest, just behind the sternum. (Some T cells mature in the tonsils.)

There are many different types of T cells, and they perform a range of functions as part of adaptive cell-mediated immunity. T cells help B cells make antibodies against invading bacteria, viruses, or other microbes.

Unlike B cells, some T cells engulf and destroy pathogens directly after binding to the antigen on the surface of the microbe.

NK T cells, not to be confused with NK cells of the innate immune system, bridge the adaptive and innate immune systems. NK T cells recognize antigens presented in a different way from many other antigens, and they can perform the functions of T helper cells and cytotoxic T cells. They can also recognize and eliminate some tumor cells.

NK cells

These are a type of lymphocyte that directly attack cells that a virus has infected.

A bone marrow transplant is useful for various reasons. For example:

• It can replace diseased, nonfunctioning bone marrow with healthy functioning bone marrow. This is useful in conditions such as leukemia, aplastic anemia, and sickle cell anemia.
• It can regenerate a new immune system that fights existing or residual leukemia or other cancers that chemotherapy or radiation therapy has not killed.
• It can replace bone marrow and restore its usual function after a person receives high doses of chemotherapy or radiation therapy to treat a malignancy.
• It can replace bone marrow with genetically healthy, functioning bone marrow to prevent further damage from a genetic disease process, such as Hurler’s syndrome or adrenoleukodystrophy.

Stem cells mainly occur in four places:

• an embryo
• bone marrow
• peripheral blood, which is present in blood vessels throughout the body
• cord blood, which is present in the umbilical cord and collectible after birth

Stem cells for transplantation are obtainable from any of these except the fetus.

Hematopoietic stem cell transplantation (HSCT) involves the intravenous (IV) infusion of stem cells collected from bone marrow, peripheral blood, or umbilical cord blood.

This is useful for reestablishing hematopoietic function in people whose bone marrow or immune system is damaged or defective.

Worldwide, more than 50,000 first HSCT procedures, 28,000 autologous transplantation procedures, and 21,000 allogeneic transplantation procedures take place every year. This is according to a 2015 report by the Worldwide Network for Blood and Marrow Transplantation.

This number continues to increase by over 7% annually. Reductions in organ damage, infection, and severe, acute graft-versus-host disease (GVHD) seem to be contributing to improved outcomes.

In a study of 854 people who survived at least 2 years after autologous HSCT for hematologic malignancy, 68.8% were still alive 10 years after transplantation.

Bone marrow transplants are the leading treatment option for conditions that threaten bone marrow’s ability to function, such as leukemia.

A transplant can help rebuild the body’s capacity to produce blood cells and bring their numbers to acceptable levels. Conditions that may be treatable with a bone marrow transplant include both cancerous and noncancerous diseases.

Cancerous diseases may or may not specifically involve blood cells, but cancer treatment can destroy the body’s ability to manufacture new blood cells.

A person with cancer usually undergoes chemotherapy before transplantation. This eliminates the compromised marrow.

A healthcare professional then harvests the bone marrow of a matching donor — which, in many cases, is a close family member — and ready it for transplant.

Types of bone marrow transplant

Types of bone marrow transplant include:

• Autologous transplant: People receive their own stem cells from their peripheral or cord blood to replenish bone marrow.
• Syngeneic transplant: People receive stem cells from their identical twin.
• Allogeneic transplant: People receive matching stem cells from a sibling, parent, or unrelated donor.
• Haploidentical transplantation: This is a treatment option for the approximately 70% of people who do not have a human leukocyte antigen (HLA)-identical matching donor.
• Umbilical cord blood (a type of allogeneic transplant): A healthcare professional removes stem cells from a newborn baby’s umbilical cord right after birth. They freeze and store the stem cells until they are needed for a transplant. Umbilical cord blood cells are very immature, so there is less of a need for matching, but blood counts take much longer to recover.

Tissue type

A person’s tissue type is defined as the type of HLA they have on the surface of most of the cells in their body. HLA is a protein, or marker, that the body uses to help it determine whether or not the cell belongs to the body.

To check if the tissue type is compatible, doctors assess how many proteins match on the surface of the donor’s and recipient’s blood cells. There are millions of different tissue types, but some are more common than others.

Tissue type is inherited, and types pass on from each parent. This means that a relative is more likely to have a matching tissue type.

However, if it is not possible to find a suitable bone marrow donor among family members, healthcare professionals try to find someone with a compatible tissue type on the bone marrow donor register.

Pre-transplant tests

Healthcare professionals perform several tests before a bone marrow transplant to identify any potential problems.

These tests include:

• tissue typing and a variety of blood tests
• chest X-rays
• pulmonary function tests
• CT or CAT scans
• heart function tests, including an electrocardiogram and echocardiogram (ECG)
• bone marrow biopsy
• skeletal survey

In addition, a person needs a complete dental exam before a bone marrow transplant to reduce the risk of infection. Other precautions to lower the risk of infection are also necessary before the transplant.

Harvesting bone marrow

Bone marrow is obtainable for examination by bone marrow biopsy and bone marrow aspiration.

Bone marrow harvesting has become a relatively routine procedure. Healthcare professionals generally aspirate it from the posterior iliac crests while the donor is under either regional or general anesthesia.

Healthcare professionals can also take it from the sternum or from the upper tibia in children, as it still contains a substantial amount of red bone marrow.

To do so, they insert a needle into the bone, usually in the hip, and withdraw some bone marrow. They then freeze and store this bone marrow.

National Marrow Donor Program (NMDP) guidelines limit the volume of removable bone marrow to 20 milliliters (ml) per kg of donor weight. A dose of 1 x 103 and 2 x 108 marrow mononuclear cells per kg is necessary to establish engraftment in autologous and allogeneic marrow transplants, respectively.

Complications related to bone marrow harvesting are rare. When they do occur, they typically involve problems related to anesthetics, infection, and bleeding.

Another way to evaluate bone marrow function is to give a person certain drugs that stimulate the release of stem cells from bone marrow into circulating blood.

A healthcare professional then obtains a blood sample and isolates stem cells for microscopic examination. In newborns, they may retrieve stem cells from the umbilical cord.

How do healthcare professionals transplant bone marrow?

Before the transplant, the person may receive chemotherapy, radiation therapy, or both. There are two ways of doing this: ablative (myeloablative) treatment and reduced intensity treatment, or a mini-transplant.

In ablative (myeloablative) treatment, a person receives high dose chemotherapy, radiation therapy, or both to kill any cancer cells. This also kills all healthy bone marrow that remains and allows new stem cells to grow in the bone marrow.

In reduced intensity treatment, or a mini-transplant, a person receives lower doses of chemotherapy and radiation therapy before a transplant. This allows older adults and those with other health problems to have a transplant.

A stem cell transplant usually takes place after chemotherapy and radiation therapy are complete.

The infusion of either bone marrow or peripheral blood is a relatively simple process that a healthcare professional performs at the bedside. They infuse the bone marrow product through a central vein through an IV tube over a period of several hours.

Autologous products are almost always cryopreserved. They thaw at the bedside and infuse rapidly over a period of several minutes.

After entering the bloodstream, the hematopoietic stem cells travel to the bone marrow. There, they begin to produce new white blood cells, red blood cells, and platelets in a process known as engraftment. This usually occurs 30 days after transplantation.

In most cases, there appears to be minimal toxicity. ABO-mismatched bone marrow infusions can sometimes lead to hemolytic reactions.

Dimethyl sulfoxide (DMSO) — which healthcare professionals use for the cryopreservation of stem cells — may give rise to facial flushing, a tickling sensation in the throat, and a strong taste of garlic in the mouth. Rarely, DMSO can cause bradycardia, abdominal pain, encephalopathy or seizures, and renal failure.

To lower the risk of encephalopathy, which is a brain condition that may occur with doses above 2 grams per kg per day of DMSO, healthcare professionals infuse stem cell infusions that exceed 500 ml over 2 days, and they limit the rate of infusion to 20 ml per minute.

Healthcare professionals regularly check blood counts. Complete recovery of immune function can take several months for autologous transplant recipients and 1–2 years for people receiving allogeneic or syngeneic transplants.

Blood tests will confirm that the body is producing new blood cells and that any cancer has not returned. Bone marrow aspiration can also help healthcare professionals determine how well the new bone marrow is working.

Risks

Complications associated with HSCT include both early and late effects.

Some early onset problems may include:

• mucositis
• hemorrhagic cystitis
• prolonged, severe pancytopenia
• infection
• GVHD
• graft failure
• pulmonary complications
• hepatic veno-occlusive disease
• thrombotic microangiopathy

Some late onset problems may include:

• chronic GVHD
• ocular effects
• endocrine effects
• pulmonary effects
• musculoskeletal effects
• neurologic effects
• immune effects
• infection
• congestive heart failure
• subsequent malignancy

Significant risks include increased susceptibility to infection, anemia, graft failure, respiratory distress, and excess fluid, which can lead to pneumonia and liver dysfunction.

A mismatch between donor and recipient tissues can lead to an immune reaction between the cells of the host and the cells of the graft.

When graft cells attack host cells, the result is a dangerous condition called GVHD. This can be acute or chronic and may manifest as a rash, a gastrointestinal illness, or liver disease. It is possible to lower the risk of GVHD through careful tissue matching.

Even when a donor antigen match is identical, 20–50% of recipients still develop GVHD, rising to 60–80% when only a single antigen is mismatched. Because of the danger of this complication, autologous transplants are more common.

Past studies suggest that people aged over 50 years have a higher risk of complications following bone marrow transplantation. For this reason, experts have typically advised against undergoing transplantation after this age.

However, advances in medical technology have reduced these risks. The authors of a 2013 report conclude that transplantation can be safe for people aged over 70 years, if they meet certain criteria.

There is little risk to those who donate because they generate new bone marrow to replace removed bone marrow. There is, however, a slight risk of infection, and a reaction to anesthetics can occur with any surgical procedure.

As bone marrow affects many bodily systems, a problem can result in a wide range of conditions, including cancers that affect the blood.

A number of conditions pose a threat to bone marrow because they prevent it from turning stem cells into essential cells.

Leukemia, Hodgkin disease, and other lymphoma cancers can damage bone marrow’s productive ability and destroy stem cells.

A bone marrow examination can help diagnose:

• leukemia
• multiple myeloma
• Gaucher’s disease
• unusual cases of anemia
• other hematological conditions

Doctors are treating a growing number of conditions using HSCT.

About every 3 minutes in the United States, someone receives a diagnosis of blood cancer. A bone marrow transplant is often the best chance for survival.

Around 30% of people can find a matching donor in their families, but 70% rely on bone marrow that someone unrelated has donated.

Healthcare professionals currently use autologous HSCT to treat:

• multiple myeloma
• non-Hodgkin lymphoma
• Hodgkin lymphoma
• acute myeloid leukemia
• neuroblastoma
• germ-cell tumors
• autoimmune conditions, such as lupus and systemic sclerosis
• amyloidosis

Healthcare professionals use allogeneic HSCT to treat:

• acute myeloid leukemia
• acute lymphoblastic leukemia
• chronic myeloid leukemia
• chronic lymphocytic leukemia
• myeloproliferative disorders
• myelodysplastic syndromes
• multiple myeloma
• non-Hodgkin lymphoma
• Hodgkin lymphoma
• aplastic anemia
• pure red cell aplasia
• paroxysmal nocturnal hemoglobinuria
• Fanconi’s anemia
• thalassemia major
• sickle cell anemia
• severe combined immunodeficiency
• Wiskott-Aldrich syndrome
• hemophagocytic lymphohistiocytosis
• genetic conditions relating to metabolism, such as mucopolysaccharidosis
• Gaucher’s disease, metachromatic leukodystrophies, and adrenoleukodystrophies
• epidermolysis bullosa
• severe congenital neutropenia
• Shwachman-Diamond syndrome
• Diamond-Blackfan anemia
• leukocyte adhesion deficiency

HSCT may also help treat:

• breast cancer, though this is not confirmed
• testicular cancer, in most cases of cancer that has returned after chemotherapy treatment
• some genetic immunologic or hematopoietic conditions

Bone marrow transplants are sometimes necessary after certain treatments, such as high dose chemotherapy and radiation therapy, that treat cancer. These treatments tend to damage healthy stem cells as well as destroy cancer cells.

Bone marrow tests

Bone marrow tests can help diagnose certain conditions, especially those related to blood and blood-forming organs. Testing provides information on iron stores and blood production.

Bone marrow aspiration uses a hollow needle to remove a small sample (about 1 ml) of bone marrow for examination under a microscope.

A healthcare professional usually inserts a needle into the hip or sternum in adults or into the upper part of the tibia (the larger bone of the lower leg) in children. They use suction to extract the sample.

They typically perform bone marrow aspiration when previous blood tests have indicated a need for it. It is particularly useful in providing information on various stages of immature blood cells.

There are two main types of bone marrow donation.

The first involves the removal of bone marrow from the back of the pelvic bone.

The second method, which is more common, is peripheral blood stem cell (PBSC) donation. This involves filtering stem cells directly from the blood. It is these blood stem cells, rather than bone marrow itself, that are necessary for the treatment of blood cancers and other conditions.

When an individual joins a bone marrow donation registry, they are agreeing to donate using whichever method the healthcare professional deems appropriate.

In terms of costs, the NMDP or a person’s medical insurance usually covers the expense of making a blood marrow donation. Donors never pay for donating, and they are never paid to donate.

The risk to a donor is minimal. Over 98.5% of donors make a full recovery after the procedure. With blood marrow donation, the major risk involves the use of anesthetics during the procedure.

With PBSC donation, the procedure itself — which involves filtering blood through a machine — is not dangerous.

Depending on a person’s ethnicity, the chance of finding a suitable bone marrow donor ranges from 23–77%, according to the NMDP’s Be The Match Registry.

Although 77% of white people in the U.S. can find a match on a donor registry, the same is true for only 23% of Black people. For those of mixed race, the percentage drops to 4%, as the combination of inherited features becomes more complex.

The NMDP has links with registries around the world, but there is an urgent need for more donors. Several researchers are calling for “further efforts” to overcome the barriers involved.

Who can donate bone marrow?

The following are some general guidelines for bone marrow donation as recommended by the NMDP.

The guidelines aim to protect the health and safety of both the donor and the recipient. Donors should contact their local NMDP center for specific details and to discuss donations with a healthcare team.

• To be listed in the registry, potential donors must be healthy and aged 18–60 years.
• If matched with a person needing a transplant, each donor must pass a medical examination and be infection-free before donating.
• People who use medications can usually donate bone marrow, as long as they are healthy and any medical conditions they have are under control at the time of donation.

Acceptable medications include:

• birth control pills
• thyroid medications
• antihistamines
• antibiotics
• prescription eye drops
• topical medications, such as skin creams

Antianxiety and antidepressant drugs are allowed as long as the condition is under control.

Donation is not possible:

• during pregnancy
• by anyone using nonprescription IV drugs
• if the person has had a positive blood test for hepatitis B or hepatitis C
• by those with specific medical conditions, such as most cancers and certain heart conditions

People with Lyme disease, malaria, or recent tattoos or piercings should wait at least a year before donating bone marrow.

How do healthcare professionals determine a bone marrow match?

After registering to donate, the person undergoes an HLA-typing test, which healthcare professionals use to match individuals with potential donors.

The healthcare professional then adds their HLA type to a database of potential donors, and they search the registry to try to find a match.

They compare proteins in the blood cells to see if they are similar to those of the recipient. They then contact the potential donor if there is a match.

The more similar the donor’s tissue type is to the individual’s, the better the chance of the person’s body accepting the transplant.

The World Marrow Donor Association (WMDA) is a collective database of hematopoietic cell donor registries from 55 countries. About 37.9 million potential donors and over 802,600 cord blood units were available as of April 2021. Preliminary searches through the NMDP also explore the WMDA.

What happens when donating bone marrow?

Hematopoietic stem cell donors undergo the following tests:

• medical history and physical examination
• serum creatinine, electrolyte, and liver function studies
• serologic studies for:
  • cytomegalovirus (CMV)
  • herpes viruses
  • HIV RNA
  • anti-HIV antibodies
  • hepatitis B and C viruses
  • human T cell lymphotropic virus-1/2
  • syphilis (VDRL)
• ABO blood typing
• HLA typing
• chest radiography
• ECG

In autologous donations, CMV and VDRL testing are not necessary.

Donating PBSC

Before a person can donate PBSC, they need to undergo daily injections of a medication called filgrastim in the 5 days leading up to the procedure. This medication draws stem cells from the bone marrow, so the donor has more of them circulating in their blood.

Donating PBSC involves a procedure known as apheresis. This is when a healthcare professional takes blood from the body using a catheter inserted into one arm. The blood passes through a machine, which filters out the stem cells, along with platelets and white blood cells. The remaining blood, which consists mainly of plasma and red blood cells, flows back into the body through a vein in the other arm.

The procedure is completely painless and is similar to donating plasma. Most PBSC donations can take place in one apheresis session that may take up to 8 hours. About 10% of PBSC donations require two apheresis sessions, each lasting 4–6 hours.

PBSC donation does not require anesthetics.

The filgrastim injections before donation may cause the following effects for several days:

• bone and muscle aches
• headaches
• fatigue
• nausea
• vomiting
• difficulty sleeping

However, these side effects usually stop soon after donation.

Most PBSC donors experience a full recovery within 7–10 days after the donation.

Donating bone marrow

If a person is donating bone marrow instead of PBSC, there is no need for filgrastim injections. Bone marrow donation is a surgical procedure that takes place in the operating room. It requires anesthetics and is, therefore, completely painless. The entire procedure takes 1–2 hours.

In around 96% of cases, the donor receives general anesthetics, which means that they are unconscious for the entire procedure. In a small number of cases, they receive local anesthetics, which numb the area that the healthcare professional takes bone marrow from. In this situation, the donor is awake throughout the procedure.

The person lies on their stomach. A healthcare professional makes an incision about a quarter of an inch in length on both sides of the pelvic bone. They then insert special, hollow needles into the bone, through which they draw the liquid marrow. The incisions do not usually require stitches.

After the procedure, the donor stays in a recovery room until they regain consciousness. Once they can eat, drink, and walk, they can leave.

Recovery

After bone marrow donation, the average recovery time is 20 days. Bone marrow replaces itself within 4–6 weeks.

People who donate bone marrow often experience:

• headaches
• fatigue
• muscle pain
• back or hip pain
• bruising around the incision site
• difficulty walking

These effects may persist for a few days to several weeks.

A person who donates PBSC is unlikely to experience any side effects following the donation, other than bruising at the needle site. Recovery time is almost immediate.

Outcome

The outcome of a bone marrow transplant depends on:

• the type of transplant
• how closely the cells match
• what type of condition the person has
• the person’s age and overall health
• the type and dosage of chemotherapy or radiation therapy they received before the transplant
• any complications that arise

A person whose condition is stable or in remission has a better chance of a good outcome than someone who has a transplant in a later stage or with relapsed disease. Young age at the time of transplant also improves the chance of success.

Transplants for nonmalignant conditions tend to have more favorable outcomes. For example, for sickle cell anemia, the overall 1-year survival rate is 94–97% if the donor is a relative or matched sibling or 83% if the donor is not related. The overall 3-year survival rate is 89–95% for transplants from related donors or 77% if the donor is unrelated.

Recipients with acute leukemia in remission at the time of transplant have an overall 1-year survival rate of 69–75% if the donor is related or 68% if the donor is unrelated. The overall 3-year survival rate is 49–58% for transplants from related donors or 53% if the donor is unrelated.

In recent years, there has been a decrease in complications such as infections and diseases. This means that the risk of death for recipients of bone marrow transplants dropped over 20% between 2003–2007 and 2013–2017, according to one 2020 analysis.

A bone marrow transplant may completely or partially cure a condition. If the transplant is successful, the person can go back to most regular activities as soon as they feel well enough. Full recovery usually takes up to a year.