A low white blood cell count is called leukopenia. The term applies when your total white blood cell count drops below 4,000 cells per microliter of blood. A normal count falls between 4,000 and 11,000 cells per microliter, so anything under that lower threshold qualifies.
Why There Are Several Names
Leukopenia is the umbrella term, but your blood contains several types of white blood cells, and each one has its own name for when levels run low. The specific term depends on which type is affected.
- Neutropenia: Low neutrophils, the most common type of white blood cell and your body’s first responders against bacteria and viruses. This is diagnosed when neutrophil counts fall below 1,500 per microliter.
- Lymphocytopenia: Low lymphocytes, the cells responsible for producing antibodies and attacking virus-infected cells. The threshold is below 1,000 per microliter in adults. Because lymphocytes only make up 20 to 40 percent of your total white blood cells, this drop doesn’t always show up on a basic blood test.
- Monocytopenia: Low monocytes, which help break down germs and clear dead cells. This is diagnosed below 200 per microliter.
Neutropenia is by far the most clinically significant of these, since neutrophils do the heavy lifting in fighting off everyday infections. When doctors express concern about a low white blood cell count, they’re usually focused on the neutrophil number specifically.
Severity Levels of Neutropenia
Not all low counts carry the same risk. Neutropenia is graded by how far the count has dropped:
- Mild: 1,000 to 1,500 cells per microliter
- Moderate: 500 to 1,000
- Severe: below 500
- Profound: below 100
At the mild end, your infection risk is only slightly elevated. Once counts drop below 500, your body has very limited ability to fight off bacteria and fungi that it would normally handle without trouble. Profound neutropenia, below 100, is a medical emergency because even harmless bacteria already living on your skin or in your gut can cause life-threatening infections.
What Causes It
Leukopenia has a wide range of triggers. Some are temporary and resolve on their own, while others require treatment.
Chemotherapy is one of the most common causes. Cancer treatments target rapidly dividing cells, and bone marrow, where white blood cells are made, gets caught in the crossfire. Radiation therapy to large areas of the body has the same effect. Many people undergoing cancer treatment experience predictable dips in their white blood cell counts in the days following each cycle.
Viral infections can temporarily suppress white blood cell production or cause the body to use them up faster than the marrow can replace them. This is one reason a routine blood test during or just after a viral illness might show a low count that corrects itself within weeks.
Autoimmune conditions like lupus and rheumatoid arthritis sometimes cause leukopenia because the immune system mistakenly attacks its own white blood cells or the bone marrow cells that produce them. Bone marrow disorders, nutritional deficiencies (particularly vitamin B12 and folate), and certain medications can also drive counts down.
How It’s Diagnosed
Leukopenia is detected through a standard blood draw called a complete blood count, or CBC. This test reports your total white blood cell number, but it doesn’t tell you which type is low. For that, your doctor orders what’s called a blood differential, which breaks down the count into its five white blood cell types and measures each one separately. That distinction matters because the treatment and the level of concern depend entirely on which cell type is affected and how far it has dropped.
Your doctor will interpret the results alongside your symptoms, medical history, and sometimes additional tests. A single low reading doesn’t necessarily mean something is wrong. Repeat testing often clarifies whether the drop is a one-time finding or a pattern that needs investigation.
Symptoms to Watch For
Leukopenia itself doesn’t produce symptoms you can feel. You won’t notice that your count is low. What you will notice are the infections that result from a weakened immune system. These can show up as fever and chills, mouth sores or white patches inside the mouth, sore throat, a cough or shortness of breath, painful urination, diarrhea, or skin wounds that drain pus and won’t heal. Infections that would normally be mild can escalate quickly when your white blood cell count is low, so even a low-grade fever can be a significant warning sign.
How It’s Treated
Treatment depends on the cause. If a medication is driving the count down, switching to an alternative often lets the marrow recover. If a viral infection is responsible, the count typically bounces back on its own once the infection clears.
For people with neutropenia caused by chemotherapy or bone marrow conditions, doctors sometimes use a growth factor that stimulates the bone marrow to produce more neutrophils. This is given as an injection under the skin, usually starting the day after a chemotherapy session ends. Some versions require daily shots, while a longer-acting form only needs a single injection between treatment cycles. These growth factors are also used for people born with conditions that cause chronically low neutrophil counts.
While your count is low, practical precautions help reduce infection risk. The CDC notes there isn’t much you can do to prevent the count itself from dropping, but you can lower your chances of getting sick during that window. Frequent handwashing, avoiding crowds and sick contacts, cooking meats thoroughly, and steering clear of fresh-cut flowers (which can harbor mold and bacteria) are all standard recommendations. Keeping your mouth clean with gentle brushing and avoiding cuts or scrapes also helps, since broken skin is an easy entry point for germs your body can’t currently fight well.