Eosinophilic asthma is a subtype of asthma driven by high levels of eosinophils, a type of white blood cell that causes persistent inflammation in the airways. It accounts for a significant portion of severe asthma cases and tends to be harder to control with standard inhalers alone. Unlike the allergic asthma most people develop in childhood, eosinophilic asthma frequently appears in adulthood and often resists the corticosteroid treatments that work well for other forms of asthma.
How It Differs From Other Types of Asthma
Most people think of asthma as one condition, but it actually has several distinct subtypes that behave differently and respond to different treatments. Allergic asthma, the most common form, flares up in response to allergens like pollen or pet dander and involves elevated levels of an antibody called IgE. Eosinophilic asthma can overlap with allergic asthma, but it also exists independently, meaning you can have it without any identifiable allergy triggers at all.
The key distinction is what’s happening at the cellular level. In eosinophilic asthma, eosinophils accumulate in the airway tissue in abnormally high numbers, thickening the airway walls and making them chronically inflamed. This subtype tends to be more severe, with frequent flare-ups and a pattern of getting worse over time if not specifically treated. People with eosinophilic asthma often find that their symptoms don’t fully respond to inhaled corticosteroids, which is a major clue during diagnosis.
What Happens Inside the Airways
The process starts when the cells lining the airways overreact to triggers that shouldn’t normally cause problems, such as allergens, viruses, or even minor tissue irritation. These stressed airway cells release alarm signals (proteins called IL-25, IL-33, and others) that activate the immune system’s inflammatory branch. Specific immune cells then begin pumping out three key signaling molecules: IL-4, IL-5, and IL-13.
Each of these molecules plays a different role. IL-5 is the primary driver of eosinophil production. It tells the bone marrow to make more eosinophils and helps them survive longer once they’re released into the bloodstream. IL-13 triggers the airway lining to produce excess mucus and makes the airways hyperreactive, meaning they tighten and spasm more easily. It also helps pull eosinophils toward the inflamed tissue. Together, these signals create a self-reinforcing cycle: inflammation triggers eosinophil recruitment, and eosinophils cause more inflammation and tissue damage.
Over time, this ongoing process remodels the airway walls, making them permanently thicker and narrower. That’s why early identification and targeted treatment matter so much.
Symptoms and Common Patterns
The day-to-day symptoms of eosinophilic asthma look similar to other forms of asthma: wheezing, shortness of breath, chest tightness, and coughing. What sets it apart is the pattern. People with this subtype tend to experience more frequent and more severe flare-ups, sometimes requiring oral steroids or emergency care multiple times a year. Symptoms often persist even when using high-dose inhaled steroids consistently.
Eosinophilic asthma also commonly shows up alongside other conditions driven by eosinophilic inflammation. Chronic sinus problems, nasal polyps, and a reduced sense of smell are frequent companions. Some people develop a sensitivity to aspirin or other anti-inflammatory painkillers. If you have asthma that started in adulthood and you’re also dealing with recurring sinus issues, that combination is a strong signal pointing toward the eosinophilic subtype.
How It’s Diagnosed
Diagnosing eosinophilic asthma relies on measuring the level of eosinophilic inflammation through two main tests: a blood eosinophil count and a breathing test called FeNO (fractional exhaled nitric oxide).
Blood Eosinophil Count
This is a simple blood draw. A count above 300 cells per microliter is generally used to identify the eosinophilic phenotype and predict who is at higher risk for flare-ups and who will respond well to corticosteroid treatment. A threshold of 150 cells per microliter or higher is used to predict whether someone will benefit from newer biologic therapies. These numbers aren’t rigid cutoffs; your doctor will interpret them alongside your symptoms and treatment history.
FeNO Testing
FeNO measures the amount of nitric oxide in your breath, which rises when eosinophilic inflammation is active in the airways. The test is simple: you breathe slowly into a handheld device for about 10 seconds. In adults, a reading above 50 parts per billion (ppb) strongly suggests eosinophilic inflammation. Below 25 ppb, eosinophilic inflammation is unlikely. Readings between 25 and 50 ppb fall into a gray zone that needs to be interpreted alongside other findings. For children, the thresholds are slightly lower: above 35 ppb is high, and below 20 ppb is low.
Neither test alone confirms the diagnosis. Doctors typically combine blood eosinophil levels, FeNO results, symptom patterns, and treatment response to build the full picture.
Standard Treatment Approach
Initial treatment follows the same stepwise approach as other asthma: inhaled corticosteroids to reduce inflammation and bronchodilators to open the airways. Many people with mild eosinophilic asthma can be managed at this level. The challenge comes when the disease is moderate to severe, which it often is. When someone is already on high-dose inhaled steroids and a long-acting bronchodilator but still experiences frequent flare-ups, that’s when biologic therapies enter the picture.
Oral corticosteroids (like prednisone) can control eosinophilic inflammation effectively, but the side effects of long-term use, including bone thinning, weight gain, diabetes risk, and immune suppression, make them a poor long-term strategy. A major goal of newer treatments is to reduce or eliminate the need for oral steroids entirely.
Biologic Therapies
Biologic medications have transformed the treatment of severe eosinophilic asthma over the past decade. These are injectable drugs designed to block the specific signaling molecules that drive eosinophil production and airway inflammation. Three biologics currently target the IL-5 pathway and are FDA-approved specifically for severe eosinophilic asthma.
Mepolizumab, approved in 2015, binds directly to IL-5 and prevents it from stimulating eosinophil production. It’s approved for patients 6 and older. Reslizumab, approved in 2016, also targets IL-5 but is given as an intravenous infusion rather than a self-injection, and is approved for adults 18 and older. Benralizumab, approved in 2017, takes a slightly different approach: instead of blocking IL-5 itself, it targets the receptor that IL-5 binds to on eosinophils, which not only blocks the signal but also triggers the direct destruction of existing eosinophils. It’s approved for patients 12 and older.
Another biologic, dupilumab, blocks the receptors for both IL-4 and IL-13, addressing the mucus overproduction and airway hyperreactivity side of the equation. It’s used for moderate-to-severe asthma with an eosinophilic profile and is also approved for conditions like eczema and nasal polyps, which makes it a practical choice for people dealing with multiple eosinophilic conditions at once.
These medications are given as injections every two to eight weeks, depending on the drug. Most people can self-inject at home after the first few supervised doses. The results can be significant: fewer flare-ups, less reliance on oral steroids, and better day-to-day breathing. They’re not cures, though. Stopping treatment typically allows eosinophil levels and symptoms to return.
Living With Eosinophilic Asthma
Getting the right diagnosis is often the biggest hurdle. Many people spend years on standard asthma treatments that provide only partial relief before anyone checks their eosinophil levels. If your asthma developed in adulthood, requires frequent steroid bursts, or hasn’t responded well to conventional treatment, asking about eosinophil testing is a reasonable next step.
Once identified and properly treated, the outlook improves substantially. Biologic therapies have given people with severe eosinophilic asthma a level of control that wasn’t possible 15 years ago. Tracking your FeNO levels and blood eosinophil counts over time helps gauge whether your treatment is working and can guide decisions about adjusting medications. The goal is fewer flare-ups, less mucus, more open airways, and ultimately less medication overall.