A white blood cell count below 4,500 cells per microliter of blood is considered low, a condition doctors call leukopenia. The causes range from common viral infections and medication side effects to nutritional deficiencies and autoimmune diseases. In most cases, the drop is temporary and resolves once the underlying trigger is addressed.
How White Blood Cells Are Made and Lost
Your bone marrow continuously produces white blood cells, which then circulate through your bloodstream for hours to days before being naturally replaced. A low count happens through one of three basic mechanisms: your bone marrow isn’t producing enough cells, something is destroying them faster than they can be replaced, or they’re being trapped somewhere in the body (usually the spleen). Understanding which mechanism is at work helps narrow down the cause.
Viral and Bacterial Infections
One of the most common reasons for a temporarily low white blood cell count is a viral infection. Viruses like HIV, Epstein-Barr (the virus behind mono), hepatitis, influenza, and SARS-CoV-2 can all drive white blood cells down. They do this partly by triggering a self-destruct process in immune cells called apoptosis. Interestingly, the number of cells directly infected by these viruses is often far too small to explain the drop. Instead, the broader inflammatory response during infection signals many more immune cells to die off than would normally be replaced in time.
This kind of dip is usually temporary. Once the infection clears, the bone marrow ramps production back up and counts return to normal within days to weeks. HIV is an exception: without treatment, it causes a sustained, progressive decline in specific white blood cells over months and years.
Medications That Lower the Count
Chemotherapy is the most well-known cause of low white blood cells. These drugs work by killing rapidly dividing cells, and bone marrow cells divide rapidly, so they get caught in the crossfire. The drop in white cells during chemotherapy is expected and closely monitored by oncologists.
But chemotherapy is far from the only medication that can do this. Certain antibiotics (amoxicillin, cotrimoxazole, vancomycin), drugs used to treat overactive thyroid (carbimazole, thiamazole), and the antipsychotic clozapine are among the most frequently reported culprits. In a study of 203 cases of drug-induced low white cells, the top offenders were carbimazole (28 cases), amoxicillin (22 cases), and cotrimoxazole (19 cases). Clozapine carries roughly a 1% incidence, which is why patients taking it undergo regular blood monitoring.
Drug-induced drops can happen through direct toxicity to the bone marrow or through an immune reaction where the body produces antibodies that target its own white blood cells. In most cases, stopping the medication allows the count to recover.
Autoimmune Diseases
In autoimmune conditions, the immune system mistakenly attacks the body’s own tissues, and white blood cells themselves can become targets. Lupus is the most prominent example. About 98% of people with lupus produce antinuclear antibodies, which attack the genetic material inside cells. Some also produce anti-phospholipid antibodies that damage cell membranes. The result is that the immune system actively destroys the very cells meant to protect the body.
Rheumatoid arthritis and other chronic inflammatory conditions can also lower white blood cell counts, both through direct immune-mediated destruction and through the medications used to manage these diseases, many of which suppress immune function.
Bone Marrow Disorders
Since the bone marrow is the factory where white blood cells are made, any disease that damages or crowds out healthy marrow tissue can cause persistently low counts. Aplastic anemia is a rare condition where the marrow becomes so damaged it can no longer produce adequate blood cells of any type. Myelodysplastic syndromes cause the marrow to produce abnormal, poorly functioning cells. Cancers like lymphoma and multiple myeloma can physically infiltrate the marrow, leaving less room for normal cell production.
These conditions tend to cause a sustained drop rather than a temporary one, and they often affect red blood cells and platelets in addition to white blood cells.
Nutritional Deficiencies
Your bone marrow needs specific nutrients to produce and mature white blood cells. Vitamin B12 and folate are essential for cell division, and deficiencies in either can reduce white blood cell output. Copper deficiency is a less recognized but important cause. Copper serves as a key cofactor in the enzymatic processes that drive cell division and protein synthesis in the marrow. Without enough copper, the marrow produces fewer white blood cells, and the deficiency can also cause anemia and nerve damage that mimics B12 deficiency.
Copper deficiency is worth knowing about because it’s sometimes misdiagnosed as B12 deficiency or even a bone marrow disorder. It can develop after weight-loss surgery, from excess zinc supplementation (zinc competes with copper for absorption), or from long-term inadequate dietary intake.
An Enlarged Spleen
The spleen filters your blood and removes old or damaged cells. When it becomes enlarged, a condition called splenomegaly, it can become overactive and trap or destroy too many white blood cells before they complete their normal lifespan. Common causes of an enlarged spleen include liver disease (especially cirrhosis, which increases pressure in the blood vessels running through the spleen), infections like mononucleosis and tuberculosis, and autoimmune conditions like lupus and sarcoidosis.
Alcohol Use
Chronic heavy drinking suppresses bone marrow function directly. Alcohol is toxic to the rapidly dividing cells in the marrow, and it also interferes with the absorption of nutrients like B12 and folate that the marrow needs to work properly. This creates a double hit: reduced production capacity combined with nutritional deficiency.
Ethnic Variation in Normal Ranges
People of African or Middle Eastern descent often have naturally lower white blood cell counts, with neutrophil levels sometimes as low as 500 cells per microliter. This is called ethnic neutropenia, and it’s a normal genetic variation, not a sign of disease. It’s important to be aware of this because it can lead to unnecessary concern or testing if a doctor isn’t accounting for it.
What Low White Blood Cells Feel Like
A low white blood cell count itself doesn’t cause symptoms you can feel. What it does is leave you more vulnerable to infections. The warning signs to watch for are the consequences of that vulnerability: recurring fevers and chills, mouth sores that keep coming back, and cuts or wounds that become infected and drain pus instead of healing normally. If you notice a pattern of frequent or unusual infections, that’s often the first clue that something is off with your immune cell counts.
When Low Counts Become Dangerous
Not all low counts carry the same risk. A mildly low total white blood cell count (say, 3,500) with normal distribution of cell types may need monitoring but not urgent treatment. The more critical number is the neutrophil count, since neutrophils are your first-line defense against bacterial and fungal infections. A neutrophil count below 1,500 cells per microliter is classified as neutropenia. A count at or below 200 is called agranulocytosis and carries a risk of severe, life-threatening infections, including from organisms that wouldn’t normally cause problems in a healthy person. A fever at that level is a medical emergency requiring immediate hospital treatment.