Low neutrophils, a condition called neutropenia, happens when your body either stops making enough of these infection-fighting white blood cells or destroys them faster than it can replace them. A normal neutrophil count falls above 1,500 cells per microliter of blood. Dropping below that threshold is considered neutropenia, and the causes range from common viral infections and medication side effects to rarer genetic and autoimmune conditions.
How Low Is Too Low?
Not all neutropenia carries the same risk. A count between 1,000 and 1,500 generally doesn’t weaken your immune defense, though it may prompt your doctor to look for an underlying cause. Between 500 and 1,000, infection risk rises slightly, especially if other parts of your immune system aren’t working well either. A count between 200 and 500 puts most people at meaningfully higher risk of infection.
Below 200 is the most dangerous range, sometimes called agranulocytosis. At this level, you’re vulnerable to severe, life-threatening infections, including from organisms that wouldn’t normally cause illness in a healthy person. A fever of 101°F or higher (or 100.4°F lasting at least an hour) combined with a neutrophil count under 1,500 is considered a medical emergency called febrile neutropenia, which requires antibiotics within an hour.
Viral Infections
One of the most common reasons for a temporarily low neutrophil count is a viral infection. Your body redirects immune resources during a fight against a virus, and some viruses directly suppress the bone marrow’s ability to produce new neutrophils. The usual culprits include Epstein-Barr virus (the cause of mono), influenza, HIV, parvovirus B19, and cytomegalovirus. In most of these cases, the drop is transient. Once the infection clears, neutrophil counts bounce back on their own within a few weeks.
Medications That Lower Neutrophils
Drug-induced neutropenia is one of the more common causes, and the list of potential offenders is surprisingly broad. Some medications suppress the bone marrow directly, while others trigger an immune reaction that destroys neutrophils in the bloodstream.
- Thyroid medications like methimazole and propylthiouracil, used for overactive thyroid
- Certain antibiotics, including nafcillin, penicillin G, and oxacillin
- Antiviral drugs such as acyclovir, ganciclovir, and oseltamivir (Tamiflu)
- Anti-inflammatory drugs used for conditions like rheumatoid arthritis or ulcerative colitis, including methotrexate, sulfasalazine, and leflunomide
- Antipsychotic medications, particularly clozapine and chlorpromazine
- Heart rhythm medications like quinidine and procainamide
- Skin medications such as dapsone and isotretinoin
Clozapine is one of the best-known examples. Patients taking it are required to have regular blood monitoring specifically because of the neutropenia risk. If you’ve recently started a new medication and your blood work shows low neutrophils, that connection is one of the first things worth investigating.
Nutritional Deficiencies
Your bone marrow needs specific raw materials to build neutrophils, and running low on certain nutrients can stall production. The three most important ones are vitamin B12, folate, and copper.
Vitamin B12 deficiency is the most common nutritional cause. It disrupts how the marrow produces blood cells, often lowering neutrophils alongside red blood cells. The good news is that it’s highly reversible. In documented cases, neutrophil counts returned to normal within 7 to 10 days of starting B12 supplementation.
Copper deficiency is less well known but can cause neutropenia that looks almost identical to a bone marrow disorder on lab tests. It sometimes occurs in people taking high doses of zinc supplements, since zinc blocks copper absorption. Neutrophil counts have been shown to recover within just 3 days of copper replacement. If you’ve had gastric bypass surgery, follow a restricted diet, or take zinc supplements regularly, copper deficiency is worth considering.
Autoimmune Causes
In autoimmune neutropenia, your immune system produces antibodies that mistakenly target and destroy your own neutrophils. This comes in two forms.
Primary autoimmune neutropenia occurs mostly in infants and young children. The antibodies typically attack a specific protein on the neutrophil surface called FcγRIIIb. Despite sometimes very low counts, children with this condition often do surprisingly well because their other immune defenses compensate. Most outgrow it.
Secondary autoimmune neutropenia shows up in adults and is linked to other autoimmune diseases. Rheumatoid arthritis is one of the most common associations, particularly a combination called Felty’s syndrome, where rheumatoid arthritis occurs alongside an enlarged spleen and low neutrophils. Lupus is another frequent cause: the antibodies in lupus can attack neutrophils directly and may also target the stem cells in the bone marrow that produce them, hitting production and survival at the same time. Other conditions associated with secondary autoimmune neutropenia include Sjögren’s syndrome, primary biliary cirrhosis, systemic sclerosis, and multiple sclerosis.
Bone Marrow Disorders
Because neutrophils are manufactured in the bone marrow, any disease that damages or crowds out normal marrow function can cause neutropenia. Aplastic anemia, where the marrow stops producing enough blood cells of all types, is one of the more serious examples. Myelodysplastic syndromes, a group of conditions where the marrow produces defective blood cells, also commonly cause low neutrophils. Leukemia and other cancers that invade the marrow can physically displace the cells responsible for neutrophil production.
Chemotherapy and radiation therapy for cancer are among the most predictable causes of neutropenia. These treatments target rapidly dividing cells, and the marrow’s blood-forming cells divide quickly, making them collateral damage. This is why oncologists monitor blood counts closely during treatment and why infections are a major concern for cancer patients.
Severe Congenital Neutropenia
Some people are born with genetic mutations that prevent their bone marrow from producing neutrophils normally. Severe congenital neutropenia (SCN) is the most direct example: the marrow simply fails to make adequate neutrophils from birth. These children face recurrent serious infections starting in infancy and typically need ongoing treatment to stimulate neutrophil production.
Benign Ethnic Neutropenia
This is an important cause to understand because it’s not a disease at all. People of African, Middle Eastern, and certain other ancestries commonly have neutrophil counts that fall below the standard 1,500 threshold without any increased risk of infection or illness. This is called benign ethnic neutropenia, and it affects a significant portion of these populations.
The cause is a well-characterized genetic variant in the DARC gene on chromosome 1. People with this variant have what’s known as the Duffy-null blood type, which results in fewer neutrophils circulating in the bloodstream (though more may be stored in the bone marrow and tissues). The variant accounts for about two-thirds of the Duffy-null trait in African Americans and up to 100% in West Africans. It has also been identified in Yemenite Jewish populations. Fewer than 3% of white individuals carry a Duffy-null phenotype, and in those cases it stems from different genetic changes.
If you’re of African or Middle Eastern descent and your blood work has flagged a mildly low neutrophil count, benign ethnic neutropenia is a likely explanation. It doesn’t require treatment and doesn’t increase infection risk. The main practical concern is making sure it isn’t mistakenly treated as a sign of disease, which can lead to unnecessary testing or delays in receiving medications that list neutropenia as a contraindication.
How the Cause Is Identified
Figuring out why your neutrophils are low usually starts with context. Your doctor will look at whether the drop is new or chronic, whether other blood cell types are also low, what medications you’re taking, and whether you’ve been sick recently. A single low reading after a viral illness often just needs a repeat blood test in a few weeks to confirm the count has recovered.
If neutropenia persists, the diagnostic process gets more involved. Blood tests can check for nutritional deficiencies (B12, folate, copper), autoimmune markers, and signs of infection like HIV. If these don’t explain the problem, a bone marrow biopsy may be needed to look at the factory itself, distinguishing between a production problem (the marrow isn’t making enough) and a destruction problem (neutrophils are being made but eliminated in the bloodstream). Your ethnic background, family history, and the severity of the drop all help narrow the possibilities.