The most common reasons for a low white blood cell count are viral infections, medications, and autoimmune diseases. A normal white blood cell count falls between 4,500 and 11,000 cells per microliter of blood. When your count drops below 4,500, the condition is called leukopenia, and it means your body has fewer immune cells available to fight off threats.
Most of the time, leukopenia is driven by a drop in one specific type of white blood cell: neutrophils, which are your body’s first responders against bacteria and fungi. That’s why low white blood cell counts and a related condition called neutropenia often go hand in hand.
Viral Infections and Bone Marrow Suppression
Viral infections are one of the most frequent triggers of a temporarily low white blood cell count. When a virus enters your body, your immune system deploys large numbers of white blood cells to fight it off. This rapid consumption can outpace your bone marrow’s ability to replace them, causing a short-term dip. Some viruses go a step further: they can directly infect the blood-forming cells inside your bone marrow, disrupting production at the source. HIV is a well-known example of a virus that causes sustained bone marrow suppression, but even common infections like the flu can cause a temporary drop.
In most cases, white blood cell counts recover on their own once the infection clears. If a virus is responsible, treatment focuses on resolving the underlying infection with antiviral drugs when appropriate.
Medications That Lower White Blood Cells
A wide range of medications can suppress white blood cell production as a side effect. Chemotherapy is the most well-known culprit. Cancer treatment works by killing rapidly dividing cells, and because bone marrow cells divide quickly to keep producing blood cells, they’re caught in the crossfire. Leukopenia is one of the most common and expected side effects of chemotherapy, and it’s a major reason cancer patients are so vulnerable to infections during treatment.
But chemotherapy isn’t the only drug class that causes this. Antibiotics, anti-seizure medications, antihistamines, antithyroid drugs, barbiturates, diuretics, and a group of antibiotics called sulfonamides can all lower white blood cell counts. If you’re on a long-term medication and your bloodwork shows a low count, the drug itself may be the explanation. In many cases, counts recover after the medication is stopped or adjusted.
Autoimmune Diseases
In autoimmune conditions, your immune system mistakenly attacks healthy tissue, and sometimes that includes your own white blood cells. Lupus and rheumatoid arthritis are the autoimmune diseases most commonly linked to low white blood cell counts. In these cases, the body produces antibodies that target and destroy neutrophils, a process called autoimmune neutropenia.
This form of low white blood cell count tends to be chronic rather than temporary. It often shows up on routine bloodwork before causing noticeable symptoms, and managing the underlying autoimmune disease is typically the main treatment approach.
Bone Marrow Disorders and Blood Cancers
Your bone marrow is the factory where all blood cells are made, so any disease that damages or disrupts it can lead to low white blood cell counts. Leukemia is a key example. In leukemia, the bone marrow begins producing abnormal blood cells that multiply uncontrollably. These abnormal cells crowd out healthy white blood cells, red blood cells, and platelets, leaving the body with fewer functional immune cells than it needs.
Other bone marrow disorders, including aplastic anemia and myelodysplastic syndromes, can also reduce white blood cell production. These conditions tend to affect multiple blood cell types at once, so a low white blood cell count may show up alongside anemia or easy bruising.
Nutritional Deficiencies
Your bone marrow needs specific nutrients to produce white blood cells efficiently. Deficiencies in vitamin B12 and folate are known to cause both lower white blood cell numbers and impaired function of the cells that are produced. In vitamin B12 deficiency specifically, the white blood cells that do get made are less effective at killing bacteria, with their infection-fighting capacity reduced to roughly 35% of normal levels. Folate deficiency affects cell counts but appears to leave cell function more intact.
These deficiencies are correctable with supplementation, and white blood cell counts typically improve once nutrient levels are restored.
Leukopenia vs. Neutropenia
You may see both of these terms on lab results, and they overlap but aren’t identical. Leukopenia refers to a total white blood cell count below 4,000 per microliter. Neutropenia specifically means the neutrophil count has dropped below 1,500 per microliter. Since neutrophils make up the largest share of your white blood cells, neutropenia is usually the main driver of leukopenia.
Neutropenia is graded by severity:
- Mild: 1,000 to 1,500 neutrophils per microliter
- Moderate: 500 to 1,000
- Severe: below 500
- Profound: below 100
One important note: people of African or Middle Eastern descent often have naturally lower neutrophil counts, sometimes as low as 500 per microliter, without any increased infection risk. This is called ethnic neutropenia and is a normal variation, not a disease.
How Low White Blood Cells Affect You
Leukopenia itself doesn’t cause symptoms you can feel. You won’t notice your white blood cell count dropping. What you will notice are the consequences: infections that come more often, last longer, or hit harder than expected. A cut that won’t heal, a fever that keeps returning, mouth sores, or recurring sinus or urinary tract infections can all be signs that your immune system is understaffed.
The risk of serious infection rises sharply as neutrophil counts fall below 500. At that level, even bacteria that normally live harmlessly on your skin or in your gut can cause dangerous infections because there aren’t enough white blood cells to keep them in check. This is why people undergoing chemotherapy are often monitored with frequent blood tests and may need to take extra precautions to avoid exposure to germs during their lowest-count periods.
How the Cause Is Identified
A single low reading on a blood test doesn’t necessarily mean something is wrong. Temporary dips are common during viral illnesses, and counts can fluctuate throughout the day. If a low count shows up, the next step is usually a repeat test to see if the number stays low. From there, your doctor will look at the bigger picture: which types of white blood cells are low, whether other blood cell types are also affected, what medications you’re taking, and whether you have symptoms of infection or an underlying condition.
If the cause isn’t obvious from your history and initial bloodwork, further testing might include checking vitamin levels, screening for autoimmune antibodies, or in some cases, a bone marrow biopsy to look at how blood cells are being produced at the source.