Eosinophils

Eosinophils are specialized white blood cells that make up about 1-4% of circulating white blood cells. Named for their bright pink-red granules when stained with eosin dye, these cells serve two primary functions: defending against parasitic infections (particularly worms) and participating in allergic inflammatory responses. While beneficial for fighting parasites, eosinophils can cause tissue damage when activated inappropriately in allergic diseases.

Why does this matter? Elevated eosinophils (eosinophilia) is a common finding that prompts important clinical questions: Is there an allergic condition? Could there be a parasitic infection? Is an eosinophilic disorder present? The pattern and degree of eosinophilia, combined with clinical context, guides evaluation. Understanding your eosinophil count helps assess allergic disease activity, screen for parasitic infections, and monitor conditions where eosinophils play a pathogenic role.

Eosinophil testing provides valuable insight into allergic and parasitic conditions. Whether managing asthma, investigating unexplained symptoms, or monitoring eosinophilic diseases, knowing your eosinophil count informs clinical decisions and treatment approaches.

Order Your CBC with Differential

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Key Benefits of Testing

Eosinophil testing helps identify allergic conditions, parasitic infections, and eosinophilic disorders. Elevated counts trigger evaluation for treatable causes and help monitor disease activity in conditions like asthma, eosinophilic esophagitis, and hypereosinophilic syndromes.

For patients with allergic diseases, eosinophil counts can guide treatment decisions. New biologic therapies targeting eosinophils are transforming treatment of severe asthma and other eosinophilic conditions, making accurate eosinophil monitoring increasingly important.

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What Does This Test Measure?

Eosinophil testing measures the number and percentage of eosinophils in blood as part of the complete blood count (CBC) with differential. Both absolute eosinophil count (AEC) and percentage are reported.

What Eosinophils Are

Physical characteristics:

• Make up 1-4% of white blood cells
• Bi-lobed nucleus (looks like glasses or figure-8)
• Large granules that stain bright pink-red with eosin dye
• Slightly larger than neutrophils

Origin and lifecycle:

• Produced in bone marrow
• Maturation driven by IL-5 (key cytokine)
• Circulate briefly in blood (8-12 hours)
• Migrate to tissues where they survive days to weeks
• Concentrate in tissues with environmental exposure (gut, lungs, skin)

What Eosinophil Granules Contain

Eosinophil granules contain potent toxic proteins:

Major Basic Protein (MBP):

• Most abundant granule protein
• Toxic to parasites
• Also damages host tissues (causes tissue damage in asthma)

Eosinophil Cationic Protein (ECP):

• Toxic to parasites and bacteria
• Contributes to tissue damage

Eosinophil Peroxidase (EPO):

• Generates toxic oxidative products
• Kills pathogens

Eosinophil-Derived Neurotoxin (EDN):

• Antiviral activity
• Can damage nerve tissue

What Eosinophils Do

Parasite defense:

• Primary role: Fighting parasitic worms (helminths)
• Too large to phagocytose — eosinophils release granule contents externally
• Toxic proteins damage parasite surfaces
• Work with IgE antibodies to target parasites

Allergic inflammation:

• Recruited to sites of allergic reactions
• Release inflammatory mediators
• Contribute to late-phase allergic response
• Cause tissue damage in chronic allergic diseases
• Key players in asthma, eosinophilic esophagitis, atopic dermatitis

Immune regulation:

• Produce cytokines that shape immune responses
• May have roles in tissue remodeling and repair
• Interact with other immune cells

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Why This Test Matters

Identifies Allergic Conditions

Eosinophilia is common in allergic diseases:

• Allergic rhinitis (hay fever)
• Asthma (especially allergic/eosinophilic phenotype)
• Atopic dermatitis (eczema)
• Food allergies
• Drug allergies

Eosinophil count helps confirm allergic etiology and monitor disease control.

Screens for Parasitic Infections

Eosinophilia is a hallmark of parasitic worm infections:

• Should be considered in any patient with eosinophilia
• Especially important with travel history to endemic areas
• May be the first clue to otherwise asymptomatic infection

Detects Eosinophilic Disorders

Several conditions feature pathologic eosinophil accumulation:

• Eosinophilic esophagitis (EoE)
• Eosinophilic gastroenteritis
• Eosinophilic pneumonia
• Hypereosinophilic syndrome
• Churg-Strauss syndrome (EGPA)

Guides Asthma Treatment

Blood eosinophil count helps phenotype asthma:

• Eosinophilic asthma responds well to corticosteroids
• High eosinophils may indicate need for biologic therapy
• Anti-IL-5 biologics (mepolizumab, benralizumab) target eosinophils
• Eosinophil count guides therapy selection

Monitors Treatment Response

In eosinophilic conditions, tracking eosinophil count assesses:

• Response to corticosteroids
• Effectiveness of biologic therapies
• Disease activity
• Need for treatment adjustment

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What Can Affect Eosinophil Levels?

Causes of ELEVATED Eosinophils (Eosinophilia)

Allergic conditions (most common in developed countries):

• Asthma
• Allergic rhinitis
• Atopic dermatitis
• Food allergies
• Drug allergies/reactions
• Urticaria (hives)

Parasitic infections (most common worldwide):

• Roundworms (Ascaris)
• Hookworm
• Strongyloides
• Toxocara (visceral larva migrans)
• Trichinella
• Schistosomiasis
• Filariasis
• Many other helminths

Eosinophilic disorders:

• Eosinophilic esophagitis (EoE)
• Eosinophilic gastroenteritis
• Eosinophilic pneumonia
• Hypereosinophilic syndrome (HES)
• Eosinophilic granulomatosis with polyangiitis (EGPA/Churg-Strauss)

Skin conditions:

• Eczema/atopic dermatitis
• Bullous pemphigoid
• Dermatitis herpetiformis
• Drug eruptions

Autoimmune/inflammatory:

• Inflammatory bowel disease
• Celiac disease
• Connective tissue diseases (some)

Malignancies:

• Hodgkin lymphoma
• Some non-Hodgkin lymphomas
• Certain solid tumors
• Chronic eosinophilic leukemia

Other causes:

• Adrenal insufficiency
• Radiation therapy
• Cholesterol embolism
• Post-splenectomy
• Medications (many drugs can cause eosinophilia)

Causes of DECREASED Eosinophils (Eosinopenia)

Low eosinophils are less clinically significant but occur with:

• Acute bacterial infection (eosinophils shift to neutrophil production)
• Corticosteroid therapy (powerful eosinophil suppressant)
• Acute stress response
• Cushing’s syndrome
• Acute inflammation

Normal Variations

• Time of day: Lowest in morning, highest at night
• Seasons: May be higher during allergy season
• Age: Slightly higher in young children

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When Should You Get Tested?

Allergic Symptoms

• Chronic nasal congestion, sneezing, itchy eyes
• Recurrent wheezing or asthma symptoms
• Chronic skin rashes or eczema
• Suspected food allergies

Suspected Parasitic Infection

• Travel to endemic areas
• Unexplained GI symptoms with travel history
• Exposure risk (contaminated food/water, soil contact)
• Immigration from endemic regions

Unexplained Eosinophilia

When elevated eosinophils found incidentally, evaluation for underlying cause is warranted.

GI Symptoms

• Difficulty swallowing (dysphagia) — think EoE
• Food impaction
• Chronic abdominal pain
• Unexplained GI symptoms

Asthma Management

• Determining asthma phenotype
• Assessing eligibility for biologic therapy
• Monitoring response to treatment

Monitoring Eosinophilic Conditions

Regular monitoring for patients with known eosinophilic disorders.

Order Your Test

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Understanding Your Results

Eosinophil results include absolute eosinophil count (AEC) and percentage:

Eosinophil Count Interpretation

Normal eosinophils:

• Typically 1-4% of white blood cells
• No immediate concern
• Normal immune surveillance

Mild eosinophilia:

• Common finding
• Often allergic conditions
• May not require extensive workup if explanation clear

Moderate eosinophilia:

• Warrants investigation
• Consider parasites, allergies, eosinophilic disorders
• Travel history important

Marked eosinophilia:

• Requires thorough evaluation
• Parasitic infection high on differential
• Consider hypereosinophilic syndrome
• Evaluate for organ involvement
• May need specialist referral

Eosinophilia Workup Considerations

Key questions:

• Any allergic symptoms? (rhinitis, asthma, eczema)
• Travel history? Immigration status?
• GI symptoms? (think EoE, parasites)
• Skin manifestations?
• New medications?
• Constitutional symptoms? (think malignancy)

Eosinophilia + Other Findings

Eosinophilia + Basophilia: Think allergic condition

Eosinophilia + Elevated IgE: Allergic disease or parasitic infection

Eosinophilia + Travel history: Strong consideration for parasites

Eosinophilia + Dysphagia: Think eosinophilic esophagitis

Eosinophilia + Asthma + Sinusitis: Consider EGPA (Churg-Strauss)

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What to Do About Abnormal Results

For Elevated Eosinophils

Mild eosinophilia with clear allergic cause:

• Treat underlying allergic condition
• Monitor for improvement
• May not need extensive workup

Eosinophilia without clear cause:

• Detailed history (allergies, travel, medications, symptoms)
• Stool studies for ova and parasites
• Consider serologic testing for specific parasites
• IgE level
• Evaluate for organ involvement if marked elevation
• Consider specialist referral (allergy, infectious disease, hematology)

Marked or persistent eosinophilia:

• Thorough parasitic workup
• Evaluate for hypereosinophilic syndrome
• Screen for end-organ damage (heart, lungs)
• Consider bone marrow evaluation if very high or unexplained
• Specialist referral recommended

For Low Eosinophils

• Usually not clinically significant
• Expected with acute infection or corticosteroid use
• Rarely requires specific evaluation

Treatment Considerations

For allergic conditions: Treat underlying allergy; eosinophils will improve

For parasites: Appropriate antiparasitic therapy based on specific organism

For eosinophilic disorders: Corticosteroids often first-line; biologics (anti-IL-5) for severe/refractory cases

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Related Health Conditions

Asthma

Eosinophilic Phenotype: Many asthmatics have eosinophilic airway inflammation. Blood eosinophil count helps identify this phenotype and guides treatment with corticosteroids and eosinophil-targeting biologics. Learn more →

Eosinophilic Esophagitis (EoE)

Swallowing Difficulty: EoE is an allergic condition causing eosinophil accumulation in the esophagus, leading to difficulty swallowing and food impaction. Blood eosinophilia is common but diagnosis requires endoscopy. Learn more →

Parasitic Infections

Helminth Defense: Eosinophilia is the hallmark blood finding with parasitic worm infections. Degree of eosinophilia varies by parasite type and stage of infection. Learn more →

Hypereosinophilic Syndrome

Organ Damage: HES features persistently high eosinophils causing damage to heart, lungs, skin, and nervous system. Requires aggressive treatment to prevent complications. Learn more →

Allergic Rhinitis

Hay Fever: Seasonal or perennial allergic rhinitis commonly causes mild eosinophilia. Eosinophils contribute to nasal inflammation and symptoms. Learn more →

Atopic Dermatitis

Eczema: This chronic allergic skin condition often features blood eosinophilia reflecting systemic allergic inflammation. Learn more →

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Why Testing Matters

Eosinophil testing identifies allergic conditions and parasitic infections — both treatable causes of eosinophilia. With growing use of eosinophil-targeting biologic therapies, accurate eosinophil counts increasingly guide treatment decisions for asthma and other eosinophilic diseases. Understanding your eosinophil level helps optimize allergy management and ensures appropriate evaluation for parasites or other underlying conditions.

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Related Biomarkers Often Tested Together

Complete Blood Count (CBC) — Eosinophils are measured as part of the CBC with differential.

IgE (Immunoglobulin E) — Allergy antibody; often elevated with eosinophilia in allergic conditions.

Basophils — May be elevated alongside eosinophils in allergic states.

White Blood Cell Count — Total WBC provides context for eosinophil percentage.

Stool Ova and Parasites — When parasitic infection suspected.

Tryptase — Mast cell marker; elevated in allergic reactions and mastocytosis.

Note: Information provided in this article is for educational purposes and doesn’t replace personalized medical advice.