Eosinophils are a type of white blood cell that shows up as a line item on a standard blood test called a complete blood count (CBC). A normal count falls between about 30 and 350 cells per microliter of blood, making up less than 5% of your total white blood cells. If your results flag eosinophils as high or low, it usually signals that your immune system is reacting to something specific, from allergies to infections to medication side effects.
What Eosinophils Actually Do
Eosinophils are part of your immune system, but they have a specialized job compared to other white blood cells. They’re built to fight threats that are too large for ordinary immune cells to swallow, particularly parasitic worms and certain fungi. Inside each eosinophil are granules packed with toxic proteins that can punch holes in the outer membranes of these organisms and kill them on contact.
Beyond fighting parasites, eosinophils also play a role in everyday immune maintenance. They help regulate inflammation, support tissue repair, and contribute to your body’s defense against some bacteria and viruses. When eosinophils encounter bacteria, they can release strands of their own DNA that act like sticky traps, binding to and killing bacteria in the space between cells. So while they make up a tiny fraction of your white blood cells, they’re far from minor players.
How to Read Your Results
Your blood test may report eosinophils in two ways: as a percentage of total white blood cells and as an absolute count (the actual number of cells per microliter). The absolute count is more useful for spotting problems because the percentage can shift depending on changes in other white blood cell types without your eosinophils actually changing.
Large population studies across Austria, Japan, and China have found that the median eosinophil count in healthy, nonsmoking adults without allergies or asthma sits around 100 cells per microliter. The 95th percentile, meaning only 5% of healthy people exceed it, lands between 335 and 420 cells per microliter depending on the study. Counts above 400 are considered definitively outside the normal range. Many labs list the upper cutoff at 500, which is the traditional threshold for a diagnosis of eosinophilia.
What High Eosinophils Mean
An elevated eosinophil count, called eosinophilia, is graded by severity:
- Mild (500 to 1,500 cells/μL): Most commonly linked to allergic conditions like asthma, hay fever, eczema, food allergies, or medication reactions. Mild bacterial and viral infections can also push counts into this range.
- Moderate (1,500 to 5,000 cells/μL): More often associated with parasitic infections, certain autoimmune conditions, or some cancers.
- Severe (above 5,000 cells/μL): Raises concern for serious blood disorders, including a condition called hypereosinophilic syndrome, where persistently high levels can damage organs.
Allergies and Asthma
The most common reason for mildly elevated eosinophils is an allergic reaction. Eosinophils are central to the inflammatory response in allergic rhinitis (hay fever), eczema, asthma, and drug sensitivities. If your count is mildly high and you have a known history of allergies, that connection is often straightforward. A subtype called eosinophilic asthma is specifically defined by elevated eosinophils in the airways and blood, and identifying it helps guide treatment choices.
Parasitic Infections
Eosinophils evolved primarily to fight parasitic worms (helminths), so a worm infection can cause dramatic spikes in eosinophil counts. Common culprits include roundworms, hookworms, whipworms, and the parasites that cause schistosomiasis and trichinosis. If you’ve traveled to tropical or developing regions and your eosinophils are elevated, your doctor will likely consider parasitic testing even if you don’t have obvious digestive symptoms, since some infections are silent for months.
Medication Reactions
Certain medications can trigger eosinophil increases as a side effect. This has been documented with some seizure medications, certain antibiotics, blood thinners like warfarin, cholesterol-lowering drugs, heart medications, and some antidepressants. The rise typically resolves after stopping or switching the medication.
Hypereosinophilic Syndrome
When eosinophil counts stay above 1,500 cells per microliter for six months or longer with no identifiable allergic or parasitic cause, and there’s evidence of organ damage, doctors consider hypereosinophilic syndrome (HES). This is rare but serious. The eosinophils themselves cause the damage by infiltrating tissues, particularly the heart, lungs, skin, spleen, and nervous system. Cardiac involvement is the biggest concern because it can lead to scarring of the heart muscle. Diagnosis requires ruling out all other causes and typically involves bone marrow testing.
What Low Eosinophils Mean
A low eosinophil count, called eosinopenia, is less commonly flagged but still meaningful. Since normal eosinophils already make up just 1% to 3% of your white blood cells, they can drop to near zero quickly under certain conditions.
The most common triggers are acute bacterial infections and physical stress. When your body mounts a stress response, hormones like cortisol and adrenaline suppress eosinophil production and push existing eosinophils out of the bloodstream. In intensive care settings, an eosinophil count below 50 cells per microliter has been found to be a fairly reliable marker of sepsis, correctly identifying infection in about 80% of cases. Steroid medications (which mimic cortisol) also predictably lower eosinophil counts, so if you’re taking prednisone or a similar drug, a low reading is expected rather than alarming.
Outside of acute illness or steroid use, a low eosinophil count on its own is rarely a concern. It doesn’t typically point to a deficiency or a condition that needs separate treatment.
What Happens After an Abnormal Result
A single abnormal eosinophil count rarely leads to a diagnosis on its own. Your doctor will look at the number in context: your symptoms, other blood test values, medication list, travel history, and allergy history. A mildly elevated count in someone with seasonal allergies usually needs no further workup. A persistently elevated count without an obvious explanation typically leads to repeat testing, sometimes followed by stool samples (to check for parasites), allergy testing, or imaging studies depending on what symptoms are present.
If your count is in the moderate to severe range, or if it stays elevated after repeat testing, more specialized evaluation may include bone marrow analysis to check for blood disorders. The timeline for follow-up varies. Mild elevations might be rechecked in a few weeks or months, while severely elevated counts with symptoms prompt faster investigation.