A high eosinophil count, called eosinophilia, means you have more than 500 eosinophils per microliter of blood. Eosinophils are white blood cells that normally make up a small fraction of your immune system, and when their numbers climb, it usually signals that your body is reacting to an allergen, an infection, or less commonly, a more serious underlying condition.
What Eosinophils Do
Eosinophils are part of your innate immune system. Their original job is defending against parasitic worms. They latch onto parasites and release toxic proteins that kill them. But eosinophils also play a role in allergic reactions, where they contribute to inflammation in tissues like the airways, skin, and digestive tract. They can respond to viral and bacterial threats too, though parasites and allergens are the triggers most strongly linked to rising eosinophil levels.
Most eosinophils don’t circulate in your blood. They concentrate in tissues that interface with the outside world: your lungs, gut, and urinary tract. The number measured on a blood test is just a snapshot of what’s circulating, not the full picture of eosinophil activity in your body.
Mild, Moderate, and Severe Ranges
A normal absolute eosinophil count (AEC) is below 500 cells per microliter. Above that threshold, eosinophilia is classified into three tiers:
- Mild: 500 to 1,500 cells per microliter
- Moderate: 1,500 to 5,000 cells per microliter
- Severe: above 5,000 cells per microliter
Mild eosinophilia is common and often tied to allergies or a recent infection. Moderate and severe levels warrant closer investigation because prolonged elevation can damage organs. A separate condition called hypereosinophilic syndrome (HES) is defined by counts above 1,500 that persist for more than a month and cause organ damage, though this is relatively rare.
How It’s Measured
Your eosinophil count comes from a standard complete blood count (CBC) with differential, which is a routine blood draw. The lab either counts eosinophils directly or calculates them by multiplying your total white blood cell count by the percentage of eosinophils. No fasting or special preparation is needed.
One thing to keep in mind: certain medications and temporary conditions can mask the true count. Corticosteroids suppress eosinophils, so if you’re taking prednisone or a similar drug, your result may look artificially low. Bacterial infections can also temporarily push eosinophils down. If your doctor suspects eosinophilia, they’ll typically repeat the test to confirm the count is persistently elevated rather than a one-time blip.
Most Common Causes
Allergic conditions are the leading cause of eosinophilia in developed countries. Asthma, hay fever, eczema, and food allergies all trigger an immune cascade that recruits eosinophils into affected tissues. In eosinophilic asthma specifically, exposure to allergens like pollen, dust mites, or pet dander sets off a chain reaction: your immune system produces signaling molecules that stimulate eosinophil growth and pull them from the bloodstream into the airways, where they drive chronic inflammation.
Globally, the most common cause of significant eosinophilia is parasitic worm (helminth) infection. Roundworms, hookworms, and other helminths can push eosinophil counts well into the moderate or severe range. If you’ve traveled to tropical or subtropical regions, a parasitic cause is one of the first things to rule out.
Other common triggers include:
- Drug reactions: Certain medications can cause eosinophilia, sometimes as part of a serious condition called DRESS syndrome (drug reaction with eosinophilia and systemic symptoms). Anti-seizure drugs like phenytoin and carbamazepine are the most frequent culprits, followed by allopurinol (used for gout), certain antibiotics like minocycline, and some anti-inflammatory drugs. Symptoms typically appear days to weeks after starting the medication.
- Skin conditions: Eczema, dermatitis, and other inflammatory skin diseases frequently elevate eosinophil levels.
- Autoimmune and inflammatory diseases: Conditions like eosinophilic esophagitis (chronic inflammation of the esophagus) and certain types of vasculitis are driven by eosinophil activity.
When Eosinophils Damage Organs
Eosinophils contain granules of toxic proteins designed to kill parasites. When eosinophil levels stay high for a prolonged period, those same proteins can damage your own tissues. The organs most vulnerable are the heart, lungs, skin, esophagus, and nervous system.
What this looks like depends on which organ is affected. Lung involvement causes wheezing and shortness of breath. Skin involvement produces rashes. When the esophagus is affected, you may have throat pain or difficulty swallowing. Heart inflammation, called Löffler endocarditis, can lead to blood clots, heart failure, or valve problems. Broader symptoms of organ involvement include fatigue, fevers, night sweats, weight loss, chest pain, and stomach pain.
Organ damage is most associated with persistent counts above 1,500, particularly when they reach the severe range. Mild eosinophilia from seasonal allergies, for example, rarely causes this kind of tissue injury.
Hypereosinophilic Syndrome
HES is diagnosed when eosinophil counts exceed 1,500 on at least two blood tests taken more than a month apart, there’s evidence of organ damage from the eosinophils, and no other explanation (like a parasite or drug reaction) accounts for the elevation. It’s a diagnosis of exclusion, meaning doctors rule out every other cause first.
HES can be driven by different underlying mechanisms. In some cases, a genetic mutation causes the bone marrow to overproduce eosinophils. In others, abnormal immune cells pump out signals that keep eosinophil production running. Some cases have no identifiable cause at all. The 2022 World Health Organization classification system breaks eosinophilic disorders into categories based on these mechanisms, which helps guide treatment.
How High Eosinophil Counts Are Treated
Treatment targets the underlying cause. If allergies are driving the count up, managing the allergic condition with avoidance strategies and allergy medications often brings eosinophils back to normal. If a medication is the trigger, stopping it resolves the problem, though this needs to be done under medical supervision. Parasitic infections are treated with antiparasitic drugs.
When eosinophilia itself needs direct treatment, corticosteroids are the first-line option. They’re effective at rapidly suppressing eosinophil production and calming inflammation. The challenge is that long-term corticosteroid use comes with significant side effects, and eosinophilia often returns once the dose is reduced.
For people who need ongoing eosinophil control, biologic therapies that block the signaling molecule IL-5 have changed the treatment landscape. These injectable medications, including mepolizumab and reslizumab, specifically target the signal that tells your bone marrow to produce eosinophils. In trials of patients with hypereosinophilic syndrome, 84% of those receiving mepolizumab were able to reduce their corticosteroid dose substantially, and nearly half stopped corticosteroids entirely. The catch is that eosinophilia tends to return once these biologics are discontinued, so they’re typically used as ongoing therapy.