Eosinophils are a type of white blood cell that plays a role in the body’s immune defense. Asthma is a chronic respiratory condition characterized by airway inflammation and narrowing, making breathing difficult. Common symptoms include wheezing, coughing, and chest tightness. This article explores the relationship between eosinophils and asthma, examining how these immune cells contribute to the disease’s development and progression.
What Eosinophils Are
Eosinophils are white blood cells classified as granulocytes, containing enzyme-filled granules released to fight pathogens. They develop in the bone marrow before migrating into the bloodstream and tissues like the lungs, stomach, and skin.
A primary function of eosinophils is combating multicellular parasites, such as worms, and certain infections. They also participate in allergic reactions. Eosinophils are distinguishable by their bilobed nucleus and cytoplasm filled with granules that stain pink or reddish-orange with eosin dye.
These granules contain chemical mediators, including cytotoxic proteins like major basic protein (MBP), eosinophil cationic protein (ECP), eosinophil-derived neurotoxin (EDN), and eosinophil peroxidase (EPO). When activated, eosinophils release these substances through degranulation. In healthy individuals, eosinophils typically constitute a small percentage, around 0.5% to 5%, of the total white blood cell count.
How Eosinophils Impact Asthma
In asthma, eosinophils contribute to the disease’s pathology, leading to airway inflammation and dysfunction. When activated, these cells release inflammatory mediators, including cytokines, chemokines, and toxic granule proteins. These substances initiate the inflammatory cascade within the airways, leading to asthma symptoms.
The release of cytotoxic proteins by eosinophils directly damages the epithelial cells lining the airways. This damage compromises the airway barrier, making the lungs more susceptible to irritants and allergens. Such cellular injury also contributes to airway hyperresponsiveness, meaning the airways become overly sensitive and constrict easily in response to various triggers.
Eosinophils also play a role in airway remodeling, involving structural changes in the bronchial walls that can lead to irreversible airway narrowing. Their inflammatory products stimulate mucus production, leading to increased mucus secretion and plugging within the airways, impeding airflow. The sustained presence of eosinophils drives chronic inflammation, exacerbating bronchoconstriction and causing ongoing symptoms like wheezing, coughing, and shortness of breath. Elevated eosinophil levels are a hallmark of a specific type of asthma, indicating their central role in the disease’s underlying inflammatory processes.
Identifying Eosinophilic Asthma
Diagnosing eosinophilic asthma involves specific tests to determine eosinophil involvement. A common method is a blood eosinophil count, which measures these white blood cells circulating in the bloodstream. Elevated blood eosinophil levels, typically above 150-300 cells/µL, often suggest an eosinophilic asthma phenotype.
Another diagnostic approach is the sputum eosinophil count, analyzing mucus coughed up from the lungs. This test provides a more direct measure of eosinophilic inflammation within the airways compared to a blood test. A high percentage of eosinophils in sputum, generally exceeding 2-3%, indicates eosinophilic airway inflammation and helps guide treatment decisions.
Fractional exhaled nitric oxide (FeNO) measurement also assesses airway inflammation. FeNO levels correlate with the degree of eosinophilic inflammation, as nitric oxide is produced by inflammatory cells like eosinophils. Elevated FeNO readings, often above 25 parts per billion (ppb) in adults or 20 ppb in children, suggest ongoing eosinophilic airway inflammation. Clinicians use a combination of these test results, alongside a patient’s symptoms and medical history, to identify eosinophilic asthma and tailor management strategies.
Treatment Approaches for Eosinophilic Asthma
Treatment for eosinophilic asthma often begins with inhaled corticosteroids, which reduce airway inflammation by suppressing eosinophil activity. For individuals with more severe forms, oral corticosteroids may be prescribed to control persistent inflammation. These medications broadly reduce the immune response, lowering eosinophil numbers and their inflammatory effects in the lungs.
Beyond corticosteroids, targeted therapies known as biologics have revolutionized the management of severe eosinophilic asthma. These specialized medications target key molecules involved in eosinophil production, activation, or survival. Examples include anti-IL-5 agents (e.g., mepolizumab, reslizumab) and anti-IL-5Rα agents (e.g., benralizumab), which block interleukin-5, a cytokine essential for eosinophil maturation and survival, or its receptor.
Another biologic, anti-IgE (e.g., omalizumab), targets immunoglobulin E, an antibody involved in allergic reactions that can indirectly lead to eosinophil activation. Biologics reduce eosinophil counts and activity, decreasing asthma exacerbations and improving lung function in patients with severe eosinophilic asthma. The selection of a specific biologic depends on the patient’s individual profile, including eosinophil levels and other clinical characteristics, to ensure the most effective and personalized treatment.