A low white blood cell count, called leukopenia, means your blood contains fewer than 4,500 white blood cells per microliter. Since white blood cells are your body’s primary defense against infection, a drop below this threshold can leave you more vulnerable to illness. The causes range from common and temporary (a viral infection, a medication side effect) to serious conditions that need ongoing treatment.
How a Low Count Actually Feels
A low white blood cell count doesn’t produce symptoms on its own. You won’t feel it directly. What you’ll notice instead is that you get infections more often than usual, and those infections may be harder to shake. Frequent colds, recurring fevers, mouth sores, skin infections that keep coming back, or a general feeling of being run down can all be signs that your immune system is short-staffed. Some people discover the problem only when routine bloodwork flags an unexpected result.
Infections That Lower Your Count
Viral infections are one of the most common reasons for a temporarily low white blood cell count. When certain viruses enter your body, they can suppress bone marrow production or destroy white blood cells faster than your body replaces them. HIV is probably the best-known example, as the virus directly attacks a key type of white blood cell. But other infections cause temporary drops too, including hepatitis B and C, Epstein-Barr virus (the cause of mono), and influenza. In most of these cases, the count rebounds once the infection clears.
Some bacterial infections, particularly severe or widespread ones like sepsis, can also deplete white blood cells by consuming them faster than the marrow can produce replacements. Tuberculosis and certain tick-borne illnesses fall into this category as well.
Medications Are a Leading Cause
Drugs are one of the most frequent culprits behind a low white blood cell count, and the list extends well beyond chemotherapy. Cancer treatment drugs suppress the bone marrow directly and predictably, which is why oncologists monitor blood counts closely during treatment. But dozens of non-chemotherapy medications can do the same thing, sometimes unexpectedly.
The medications most commonly linked to this problem include:
- Antithyroid drugs used to treat overactive thyroid (Graves’ disease), including propylthiouracil and methimazole. These carry roughly a 0.2% to 0.5% risk of dropping your white blood cell count.
- Clozapine, an antipsychotic used for treatment-resistant schizophrenia, which causes low white blood cells in about 1% of patients. This risk is high enough that regular blood monitoring is required for anyone taking it.
- Certain antibiotics, including amoxicillin, vancomycin, metronidazole, and sulfamethoxazole/trimethoprim.
- Anticonvulsants and immune-suppressing drugs used after organ transplants or for autoimmune conditions.
The mechanism varies. Some drugs directly suppress the bone marrow’s ability to produce new white blood cells. Others trigger an immune reaction where the body mistakenly destroys its own white blood cells. If you’ve recently started a new medication and your count drops, your doctor will likely consider the drug as a possible cause.
Autoimmune Diseases
In autoimmune conditions, the immune system turns against the body’s own tissues. Sometimes that includes white blood cells themselves. Lupus and rheumatoid arthritis are the autoimmune diseases most commonly associated with a low white blood cell count. In these cases, the body produces antibodies that target and destroy its own white blood cells or suppress their production in the bone marrow.
This form, called autoimmune neutropenia, can exist alongside the other symptoms of these diseases or sometimes appear before a full autoimmune diagnosis is made. Treating the underlying autoimmune condition often helps stabilize the blood count.
Bone Marrow Problems
Your bone marrow is the factory where white blood cells are made. When that factory malfunctions, production drops. Several conditions can damage or crowd out the marrow:
- Aplastic anemia is a rare disorder where the marrow becomes damaged and can no longer produce enough blood cells of any type, including white blood cells, red blood cells, and platelets.
- Myelodysplastic syndromes are a group of conditions where the marrow produces abnormal, poorly functioning blood cells.
- Blood cancers like lymphoma and multiple myeloma can infiltrate the marrow, physically crowding out the healthy cells that produce white blood cells.
- Cancer that has spread to the bone marrow from other parts of the body can have the same crowding effect.
Bone marrow problems tend to affect more than just white blood cells. If your red blood cells and platelets are also low, that pattern points more strongly toward a marrow issue.
Nutritional Deficiencies
Your bone marrow needs specific nutrients to produce white blood cells, and running low on them can slow production noticeably. Copper deficiency is a particularly well-documented cause of low white blood cell counts and can sometimes mimic more serious bone marrow diseases on lab tests. Vitamin B12 deficiency can also lead to low counts, sometimes alongside anemia and neurological symptoms like fatigue, tingling, or difficulty concentrating. Folate deficiency plays a smaller role but can contribute, especially when combined with other deficiencies.
These causes are worth knowing about because they’re treatable. Correcting the deficiency with supplementation typically restores the count over weeks to months. Copper deficiency in particular is easy to miss because it’s not part of standard blood panels, so it may take some detective work to identify.
An Enlarged Spleen
The spleen acts as a filter for your blood, removing old or damaged cells. When the spleen becomes enlarged (a condition called splenomegaly), it filters too aggressively, trapping and destroying white blood cells, red blood cells, and platelets in excessive numbers. Liver disease, certain infections, and some blood cancers can all cause the spleen to enlarge. The result is a low count driven not by underproduction but by overdestruction.
Race and Individual Variation
Not every low reading on a lab report signals a problem. People of African ancestry naturally have lower white blood cell and neutrophil counts than white individuals. This is a well-established biological variation, not a disease. If your count is mildly below the standard reference range but consistent over time and you’re not getting unusual infections, it may simply be your normal baseline. Doctors familiar with this variation will interpret your results accordingly rather than launching an unnecessary workup.
How Severity Is Measured
When doctors evaluate a low white blood cell count, they focus specifically on neutrophils, the most abundant type and the body’s first responders against bacterial infection. The absolute neutrophil count (ANC) determines how much risk you face:
- Mild (1,000 to 1,500): Slightly increased infection risk but rarely causes problems on its own.
- Moderate (500 to 1,000): Meaningfully increased infection risk. You may need to take precautions.
- Severe (below 500): High infection risk. Even minor infections can become dangerous quickly.
- Profound (below 100): Extremely high risk. This level requires urgent medical attention.
What Happens During a Workup
If a routine blood test reveals a low white blood cell count, your doctor won’t jump to conclusions from a single number. The first step is usually repeating the test to confirm the finding isn’t a fluke. From there, the investigation depends on how low the count is and whether other blood cell types are also affected.
A peripheral blood smear, where a technician examines your blood cells under a microscope, is one of the most informative early steps. The automated machines that run standard blood counts can sometimes misclassify cells, so a human eye confirms the machine’s reading and can spot abnormal cell shapes that point toward specific diagnoses. Your doctor will also review your medication list, ask about recent infections, and check for signs of autoimmune disease or nutritional deficiency.
If the cause remains unclear after initial testing, or if the count is very low or dropping, a bone marrow biopsy may be recommended. This involves taking a small sample of marrow, usually from the back of the hip bone, to examine how blood cells are being produced. It sounds intimidating, but it’s typically done as an outpatient procedure with local numbing and takes about 15 to 20 minutes.