A white blood cell count below 4,500 cells per microliter of blood is considered low, a condition called leukopenia. The causes range from temporary viral infections to chronic autoimmune diseases, medication side effects, and bone marrow disorders. In many cases, the drop is mild and resolves on its own, but persistent or severe drops deserve investigation because white blood cells are your primary defense against infections.
What Counts as Low
A normal white blood cell count falls between 4,500 and 11,000 cells per microliter. Anything below 4,500 qualifies as low. But the number that matters most in practice is your neutrophil count, since neutrophils are the specific type of white blood cell that fights bacterial and fungal infections. When neutrophils alone drop low, it’s called neutropenia, and severity breaks down like this:
- Mild (1,000 to 1,500 neutrophils per microliter): minimal infection risk, often resolves without treatment
- Moderate (500 to 1,000): increased susceptibility to skin and mucosal infections
- Severe (below 500): significantly higher risk of serious bacterial, fungal, and bloodstream infections
A single low reading on a routine blood test doesn’t necessarily signal a serious problem. Counts fluctuate throughout the day and can temporarily dip during a mild illness. Doctors typically look at trends over time and whether the count stays low before pursuing further testing.
Viral Infections
One of the most common reasons for a temporarily low white blood cell count is a viral infection. Influenza, hepatitis viruses, Epstein-Barr virus (the cause of mono), and enteroviruses can all suppress white blood cell production or accelerate their destruction. COVID-19 has also been documented to cause transient drops in both white blood cells and platelets during acute infection.
These viruses work through several mechanisms. Some deplete the stem cells in your bone marrow that produce new white blood cells. Others trigger your immune system to form antibodies that mistakenly target and destroy neutrophils. In most cases, the count rebounds as the infection clears, typically within a few weeks. HIV is a notable exception: it can cause a sustained low white blood cell count because the virus directly attacks immune cells over time.
Medications That Lower the Count
Drugs are one of the most frequently identified causes of low white blood cells, and chemotherapy is the most obvious culprit. Chemotherapy drugs suppress the bone marrow’s ability to produce new cells, which is why oncology patients are closely monitored for drops in their blood counts during treatment.
But chemotherapy is far from the only medication involved. Several common drug classes can trigger the immune system to destroy neutrophils or directly suppress their production:
- Antibiotics: certain penicillin-type drugs, fluoroquinolones, and others used for serious bacterial infections
- Antithyroid drugs: medications used to treat an overactive thyroid
- Anticonvulsants: some seizure medications, including carbamazepine and phenytoin
- Psychiatric medications: clozapine, used for treatment-resistant schizophrenia, is well known for this risk
Drug-induced neutropenia can develop within days or take weeks to appear. In most cases, the count recovers after the medication is stopped, though the timeline varies. If you notice frequent infections or unexplained fevers while on a new medication, a blood count check can determine whether your white cells have dropped.
Autoimmune Diseases
In autoimmune conditions, the immune system turns against the body’s own tissues, and white blood cells can become targets. Lupus and rheumatoid arthritis are two of the most common autoimmune diseases linked to low white blood cell counts. In these cases, the body produces antibodies that destroy neutrophils in the bloodstream, a process called secondary autoimmune neutropenia.
The low count isn’t always the first clue. Most people with lupus or rheumatoid arthritis are already diagnosed before a blood test reveals low white cells. But sometimes a persistently low count on routine bloodwork is what leads a doctor to investigate for an underlying autoimmune condition, especially when combined with symptoms like joint pain, rashes, or unexplained fatigue.
Bone Marrow Disorders
Your bone marrow is the factory where all blood cells are produced. When it malfunctions, the supply of white blood cells, red blood cells, and platelets can all drop simultaneously. This pattern, where multiple blood cell types are low at once, often points to a bone marrow problem rather than a single-cause issue.
Aplastic anemia is one of the more serious examples. In this condition, the bone marrow largely stops producing enough blood cells of any type. People with aplastic anemia are significantly more vulnerable to infections because of the low white blood cell count, and they also experience fatigue from low red blood cells and easy bleeding from low platelets. Other bone marrow conditions, including certain pre-cancerous changes in the marrow, can produce similar effects by disrupting the normal development of blood cells before they’re released into the bloodstream.
Nutritional Deficiencies
Your bone marrow needs specific raw materials to produce white blood cells. Deficiencies in vitamin B12, folate, copper, and zinc can all impair production. These nutritional causes are more common in people with restricted diets, digestive conditions that impair nutrient absorption, or a history of weight-loss surgery. Correcting the deficiency typically restores the count over weeks to months.
What Symptoms to Watch For
A low white blood cell count doesn’t cause symptoms on its own. What it does is make infections more likely, and those infections are what you’ll notice. The signs to pay attention to are ones that suggest your body isn’t fighting off germs effectively: fevers that come back or won’t resolve, mouth sores or gum inflammation, skin infections that heal slowly, sore throats or sinus infections that recur more often than usual, and urinary tract infections.
The more severe the drop, the more vulnerable you become. People with severe neutropenia (below 500 cells per microliter) can develop serious infections rapidly, including pneumonia and bloodstream infections from bacteria that a healthy immune system would handle without trouble.
How Doctors Investigate a Low Count
When a low white blood cell count shows up on bloodwork, the first step is usually a repeat test to confirm it isn’t a one-time fluctuation. If the count stays low, a peripheral blood smear is one of the most useful next steps. This involves examining a drop of blood under a microscope to look at the individual cells, which can reveal abnormalities in cell shape or signs of dysplasia (cells that didn’t develop properly) that point toward specific causes.
Your doctor will also review your medications, check for recent infections, and look at whether other blood cell types are affected. If red blood cells and platelets are also low, that pattern suggests the problem originates in the bone marrow rather than in the bloodstream. In that scenario, a bone marrow biopsy may be needed to examine the tissue directly and check for conditions like aplastic anemia or other marrow disorders. For many people, though, the cause turns out to be something identifiable and treatable, like a medication side effect or a resolving viral infection, and the count returns to normal without invasive testing.