Low white blood cell counts and low neutrophils most often result from something interfering with how the bone marrow produces blood cells, though infections, medications, autoimmune conditions, and nutritional shortfalls can all play a role. Neutrophils are the most abundant type of white blood cell, so when doctors say your white count is low, they frequently mean your neutrophils specifically have dropped. The medical terms are leukopenia (low white blood cells overall) and neutropenia (low neutrophils), and the two overlap so often that they tend to be investigated together. What makes the topic genuinely complicated is that the list of potential causes is long, and identifying the right one matters because the treatment is completely different depending on the trigger.
Three Basic Ways Neutrophils Drop
Before getting into specific diseases and triggers, it helps to know that there are really only three roads to a low neutrophil count. The bone marrow can fail to make enough neutrophils, existing neutrophils can get trapped somewhere outside the bloodstream (usually in the spleen), or neutrophils can be destroyed faster than they are replaced. Most causes of low white cells fall into one or more of these categories.1Elsevier. Neutropenia: etiology and pathogenesis A single disease can sometimes involve more than one of these mechanisms at the same time, which is part of why pinning down the cause requires blood tests, and sometimes a bone marrow biopsy.
Viral and Other Infections
One of the most common and least alarming reasons for a temporary dip in white cells is a viral infection. Influenza, HIV, hepatitis, Epstein-Barr virus, and even some run-of-the-mill respiratory viruses can push neutrophil counts down for days to weeks. The mechanism involves the body’s own immune signaling working against neutrophil production. When you fight off a virus, your immune system releases signaling molecules called interferons. These interferons are essential for controlling the infection, but they also shift the bone marrow’s priorities. Interferon-gamma, produced by T cells and natural killer cells during a viral response, steers the marrow toward making more monocytes at the expense of neutrophils and other cell types.2PubMed Central. Impact of Viral Infections on Hematopoiesis: From Beneficial to Detrimental Effects on Bone Marrow Output In other words, the very act of fighting the virus temporarily suppresses the production line for neutrophils.
For most people, this is self-correcting. Once the infection clears, the marrow resumes its normal output, and the neutrophil count bounces back. The picture gets more concerning with chronic infections like HIV or hepatitis C, where the immune signaling persists and the marrow stays suppressed for longer. Bacterial infections can also cause neutropenia, but by a different route: overwhelming bacterial infections sometimes consume neutrophils faster than the marrow can replenish them, leaving counts dangerously low right when you need them most.
Medications That Suppress Neutrophil Production
Chemotherapy is the most well-known culprit, but dozens of non-chemotherapy drugs can also drive neutrophils down. The list includes certain antibiotics (like trimethoprim-sulfamethoxazole), anti-seizure medications, antithyroid drugs (especially methimazole), some psychiatric medications (clozapine is a well-known offender), and nonsteroidal anti-inflammatory drugs. The severity ranges from a mild, temporary dip to a dangerous condition called agranulocytosis, where neutrophils nearly vanish.
There appear to be two main ways non-chemo drugs cause this problem. Some drugs are directly toxic to the precursor cells in the bone marrow that would normally mature into neutrophils. Others trigger an immune-mediated reaction in which the body forms antibodies against its own neutrophils or their precursors, destroying them.3SAGE Journals. Nonchemotherapy drug-induced neutropenia and agranulocytosis: could medications be the culprit? The frustrating part is that drug-induced neutropenia is not always dose-dependent. Some people react to a standard dose that millions of others tolerate without issue, which makes it hard to predict. If you are on a medication known to carry this risk, your doctor will likely order periodic blood counts to catch a drop early.
Stopping the offending drug usually allows the count to recover, though the timeline varies. Some people bounce back in a week; others take several weeks. In severe cases, doctors may use a growth factor injection (G-CSF) to speed recovery.
Autoimmune Conditions
Autoimmune diseases attack the body’s own tissues, and sometimes neutrophils end up as collateral damage or even as direct targets. Systemic lupus erythematosus (SLE, commonly known as lupus) is one of the conditions most frequently associated with low white cell counts. In lupus, the immune system can form antibodies against neutrophils themselves, leading to their premature destruction in the bloodstream. Rheumatoid arthritis is another autoimmune disease where neutrophils play a complicated dual role. In both lupus and rheumatoid arthritis, dysregulated neutrophil activation contributes to tissue damage, with neutrophil-derived reactive oxygen species and enzymes implicated in harm to cartilage in rheumatoid arthritis and blood vessels in lupus.4Frontiers in Immunology. Neutrophils in the Pathogenesis of Rheumatoid Arthritis and Systemic Lupus Erythematosus: Same Foe Different M.O.
A related condition called autoimmune neutropenia can occur on its own, without a broader autoimmune disease. In infants and young children, this is surprisingly common and usually benign; the body produces antibodies against its own neutrophils, counts drop, and the condition resolves on its own within months to a couple of years. In adults, isolated autoimmune neutropenia is rarer and tends to be more persistent. It often coexists with other autoimmune disorders, making the diagnostic picture murkier.
Felty syndrome is a specific triad of rheumatoid arthritis, an enlarged spleen, and neutropenia. The enlarged spleen traps and destroys neutrophils, combining two of the three basic mechanisms mentioned earlier. People with Felty syndrome are at increased risk of infections because their neutrophil counts stay chronically low.
Nutritional Deficiencies
Diet rarely comes to mind when people think about white blood cell counts, but certain nutrient deficiencies can absolutely suppress neutrophil production. The bone marrow is one of the most metabolically active tissues in your body; it churns out billions of cells per day. When it lacks the raw materials it needs, production falters.
Vitamin B12 and folate deficiencies are classic culprits. Both vitamins are essential for DNA synthesis, and without them the marrow produces abnormally large, dysfunctional cells. This shows up on a blood smear as megaloblastic anemia, but the effect extends beyond red blood cells to white cells and platelets too. Less well known is copper deficiency, which can cause anemia and neutropenia and sometimes looks so much like a bone marrow disease on biopsy that it gets mistaken for myelodysplastic syndrome, a type of pre-leukemia condition.5PubMed Central. Hematologic Manifestations of Nutritional Deficiencies: Early Recognition is Essential to Prevent Serious Complications Copper deficiency is uncommon, but it can show up in people who have had weight-loss surgery, take excessive zinc supplements (zinc competes with copper for absorption), or have malabsorption conditions.
The good news with nutritional causes is that they are usually reversible. Once the deficiency is corrected with supplementation or dietary changes, blood counts tend to normalize over weeks. The important thing is recognizing the deficiency in the first place, because the blood picture can look alarming before the relatively simple fix is identified.
Bone Marrow Disorders
When the bone marrow itself is diseased, low white cells and neutropenia are often part of a broader problem. Aplastic anemia is the classic example. In aplastic anemia, the stem cells in the marrow that are supposed to give rise to all blood cell types are dramatically reduced. The result is pancytopenia, a simultaneous drop in red blood cells, white blood cells (including neutrophils), and platelets.6Elsevier. Bone marrow failure: causes and complications The causes of aplastic anemia include autoimmune attack on the marrow, toxic chemical exposures, certain drugs, and inherited genetic conditions, though in many cases the cause is never identified.
Myelodysplastic syndromes are another group of bone marrow diseases where the marrow produces blood cells that are abnormal and die prematurely. Neutropenia is a frequent feature. Leukemias, especially acute leukemia, can crowd out normal blood cell production as cancerous cells take over the marrow space. Metastatic cancers from other organs can also invade the bone marrow and suppress normal blood cell production, a process sometimes called myelophthisis.
These are the diagnoses people tend to fear most when told their white count is low, and understandably so. But it is worth keeping in perspective that bone marrow diseases are far less common than viral infections and medication side effects as a cause of neutropenia. Doctors use the pattern of abnormalities, the severity of the drop, and the clinical context to decide whether a bone marrow biopsy is warranted.
Benign Ethnic Neutropenia
One cause of low neutrophil counts that deserves special mention is benign ethnic neutropenia, a condition where people of certain ethnic backgrounds, particularly those of African, Middle Eastern, or some Caribbean descent, have neutrophil counts that sit below the standard reference range without any disease being present. Estimates suggest this affects a sizable percentage of people of African ancestry. It is not a disease. The neutrophils function normally and these individuals do not face an increased risk of infection.
The clinical issue is that standard laboratory reference ranges were largely established using populations of European descent. When someone with benign ethnic neutropenia shows up in a routine blood test, they may be told their white count is “low” and subjected to unnecessary workups, or worse, have elective surgeries or medication courses delayed because a doctor is worried about the neutrophil number. Awareness of this normal variant has been growing, but it still causes confusion. Some labs now flag that their reference ranges may not apply to all ethnic groups, and updated clinical guidelines increasingly encourage doctors to consider the patient’s baseline before reacting to a number that looks abnormal on paper.
How Severe Does It Need to Be Before You Worry?
Neutrophil counts are measured in absolute numbers (often reported as the “absolute neutrophil count” or ANC on your lab results). A normal ANC is typically above about 1,500 cells per microliter of blood. Between 1,000 and 1,500 is considered mildly low and usually does not raise infection risk much. Between 500 and 1,000 is moderate neutropenia, where infection risk starts climbing. Below 500 is severe neutropenia, and the risk of bacterial and fungal infections becomes a real clinical concern. Below 200, the risk is very high.
Context matters as much as the number. A healthy person whose count dips to 1,200 during a cold and bounces back a week later does not need a hematology referral. A person whose count drops to 800 while taking a new medication needs that medication re-evaluated. A person whose count is 300 with no obvious cause needs urgent evaluation including, likely, a bone marrow biopsy. Doctors typically repeat the blood count before making major decisions, because a single low reading can be a fluke of timing, especially if drawn during an acute illness.
When Low Neutrophils Come and Go
Some people experience neutrophil counts that cycle predictably between normal and dangerously low every few weeks. This is called cyclic neutropenia, and it has a characteristic pattern: roughly every 21 days, the neutrophil count drops to a nadir, stays low for several days, and then recovers. During the low points, people are vulnerable to mouth sores, fevers, and infections.
Cyclic neutropenia is caused by mutations in a gene involved in neutrophil production, and it tends to run in families. It is rare but important to recognize because the recurring pattern distinguishes it from other causes. Doctors who are not aware of it may attribute each episode to a different cause and miss the underlying rhythm. Growth factor injections can shorten the duration and severity of the low periods for people who experience frequent infections.
Spleen-Related Neutropenia
The spleen acts as a filter for blood cells, removing old or damaged ones. When the spleen becomes enlarged, a condition called hypersplenism, it traps and destroys blood cells more aggressively than normal. This can lower neutrophils, red blood cells, and platelets simultaneously. Liver disease with portal hypertension is one of the most common causes of an enlarged spleen, which is why people with advanced liver cirrhosis often have low blood counts across the board. Other causes include certain infections (like malaria and leishmaniasis), storage diseases, and lymphomas.
Spleen-related neutropenia is a good example of the “sequestration” mechanism. The marrow is actually producing neutrophils at a normal or even increased rate, but the enlarged spleen pulls them out of circulation before they can do their job. Treatment targets the underlying cause of the spleen enlargement. In extreme cases, surgical removal of the spleen (splenectomy) may be considered, though this comes with its own long-term infection risks.
Distinguishing Isolated Neutropenia from a Broader Problem
One of the first things a doctor evaluates when neutrophils are low is whether the rest of the blood count is normal. Isolated neutropenia, where only neutrophils are low while red cells and platelets are fine, points toward a narrower set of causes: drugs, autoimmune neutropenia, benign ethnic neutropenia, or a viral infection. When neutrophils are low alongside other cell types (a pattern called pancytopenia), the investigation shifts toward bone marrow-level problems like aplastic anemia, myelodysplastic syndromes, or nutritional deficiencies affecting DNA synthesis.6Elsevier. Bone marrow failure: causes and complications
The blood smear, an actual microscopic look at the cells, adds another layer of information. Abnormally shaped white cells or immature cells circulating where they should not be can point toward a malignancy. Normal-looking but scarce neutrophils in an otherwise healthy person suggest a more benign cause. Doctors also look at the trend: is the count dropping steadily, staying flat, or fluctuating? A count that was normal six months ago and has gradually declined warrants a different urgency than a count that has sat at 1,100 for years and never caused a problem.
For people who have been told their white cell count is low, the most practical step is confirming whether the drop is new or longstanding, whether other cell lines are affected, and whether there is an obvious trigger like a recent illness or a new medication. That information narrows the possibilities considerably and helps your doctor decide whether watchful waiting, medication adjustment, or a deeper workup is the right next move.