Aplastic anemia occurs when the bone marrow stops producing enough new blood cells. In about 75% of acquired cases, no specific cause is ever identified. For the remaining cases, the triggers range from an immune system attack on the body’s own marrow to chemical exposures, viral infections, certain medications, and inherited genetic conditions.
How the Immune System Turns on Bone Marrow
The most well-understood mechanism behind acquired aplastic anemia is autoimmune. T cells, a type of white blood cell that normally fights infections, become abnormally activated and begin attacking the stem cells in bone marrow that produce all blood cells. These stem cells, called hematopoietic stem and progenitor cells, are the factory floor for red blood cells, white blood cells, and platelets. When T cells destroy them, the marrow becomes hollow and fatty instead of dense and productive.
This immune dysfunction plays out on three levels: the T cells themselves become overactive, the stem cells sustain direct injury, and the bone marrow environment (the “niche” where stem cells live and grow) deteriorates. The result is a marrow that can’t replenish blood cells at the rate the body needs. This is why immunosuppressive therapy, which dials down the immune attack, works as a treatment for many patients.
Chemical and Radiation Exposure
Benzene is the chemical most strongly linked to aplastic anemia. At high exposure levels (air concentrations above 100 parts per million), roughly 1 in 100 exposed people develop the condition. That risk drops dramatically at lower exposures of 10 to 20 parts per million, where the incidence falls to about 1 in 10,000. Benzene is found in industrial solvents, gasoline, and cigarette smoke, though occupational regulations have significantly reduced workplace exposure in many countries.
Ionizing radiation damages bone marrow directly. High doses cause outright marrow failure, while sub-lethal doses lead to marrow suppression, leaving a person immunocompromised with abnormally low blood cell counts. This is why cancer patients receiving radiation therapy sometimes develop bone marrow problems as a side effect. The damage can be temporary or permanent depending on the dose and whether it’s delivered all at once or spread out over time. Fractionated radiation (small doses given over days or weeks) allows healthy cells some chance to repair between sessions, which is why this approach is standard in cancer treatment.
Data from atomic bomb survivors at Hiroshima and Nagasaki showed that exposure to significant radiation led to higher rates of blood cancers developing two to three years later. Interestingly, studies of Chernobyl victims, who received their radiation exposure more gradually over a longer period, did not show the same spike in bone marrow failure, suggesting that how quickly the dose is delivered matters as much as the total amount.
Medications That Can Trigger Marrow Failure
Drug-induced aplastic anemia is uncommon but well-documented. The antibiotic chloramphenicol is one of the most historically recognized culprits, which is one reason its use has been restricted in many countries. Other drug categories linked to marrow failure include anti-inflammatory drugs (NSAIDs), anticonvulsants used for seizures, antithyroid medications, antirheumatic drugs, gold salts used in arthritis treatment, and certain sulfa-based antibiotics.
Drug-induced cases are unpredictable. Most people who take these medications never develop bone marrow problems. The reaction appears to depend on individual susceptibility, possibly related to how a person’s immune system and metabolism interact with the drug. This makes it impossible to predict who will be affected, though the risk is generally very low for any single medication.
Viral Infections
Several viruses can trigger aplastic anemia by either directly damaging marrow cells or setting off the kind of immune response that leads to stem cell destruction. Hepatitis viruses are among the most recognized triggers. Post-hepatitis aplastic anemia can develop weeks to months after a liver infection, sometimes even after the hepatitis itself has cleared.
Other viruses linked to the condition include Epstein-Barr (the virus behind mono), cytomegalovirus, parvovirus B19, and HIV. In these cases, the virus likely acts as a spark that ignites an autoimmune cascade in people whose immune systems are already predisposed to overreact.
Inherited Bone Marrow Failure Syndromes
A smaller subset of aplastic anemia cases are inherited rather than acquired. These genetic conditions typically show up in childhood or early adulthood and involve mutations that impair the bone marrow’s ability to maintain healthy stem cells over time.
Fanconi anemia is the most well-known inherited cause. Dyskeratosis congenita is another, caused by mutations in genes responsible for maintaining telomeres, the protective caps on the ends of chromosomes. When telomeres can’t be properly maintained, cells lose their ability to divide and replenish. About half of dyskeratosis congenita cases trace back to mutations in four specific genes (TERT, TERC, DKC1, or TINF2), with several other telomere-related genes accounting for additional cases.
Dyskeratosis congenita often comes with recognizable physical signs: poorly growing or abnormally shaped fingernails and toenails, lacy patterns of skin discoloration on the neck and chest, and white patches inside the mouth. More severe forms of the condition can affect the brain or the retina. These inherited syndromes carry a lifelong risk of bone marrow failure and also increase the risk of certain cancers.
Pregnancy as a Trigger
In rare cases, aplastic anemia develops during pregnancy. The exact mechanism isn’t fully understood, but pregnancy triggers significant immune system changes. The body adjusts its immune responses to protect the developing fetus while still fighting infections, and this period of immune flux may, in susceptible women, tip the balance toward an autoimmune attack on bone marrow. In some patients, the condition resolves after delivery, but recovery is not guaranteed. Case reports have documented patients showing no improvement even weeks after giving birth.
How Common Is Aplastic Anemia
Aplastic anemia is rare, with an annual incidence of roughly 1.5 to 7 cases per million people worldwide. The rate varies by geography: studies from Asia consistently report higher incidence (over 4 per million per year) compared to more recent data from Europe and the Americas. Earlier Western studies from the 1960s and 70s reported rates of 6 to 10 per million, but more recent figures have come down, likely reflecting better diagnostic criteria and reduced environmental exposures. The median age at diagnosis ranges widely, from 25 to 60 years, depending on the population studied.
Why Most Cases Have No Clear Cause
Despite all the known triggers, the majority of acquired aplastic anemia cases remain idiopathic, meaning no specific cause is found even after thorough investigation. About 75 out of every 100 acquired cases fall into this category. Many of these are still thought to involve autoimmune destruction of marrow stem cells, but whatever initially triggered the immune system to go rogue can’t be pinpointed. This can be frustrating for patients looking for answers, but the good news is that treatment decisions don’t depend on identifying the original cause. Whether the immune attack was sparked by a virus, a chemical, or something entirely unknown, the approach to restoring marrow function follows similar principles.