Eosinophilia, often encountered in hematological reports, is more than just a lab abnormality. It is a clinical clue to a variety of underlying diseases—from benign allergic responses to sinister systemic disorders.
To simplify learning and recall, we’ll explore the mnemonic “ALLERGIC”, which beautifully encapsulates the key causes of eosinophilia.
What is Eosinophilia?
Eosinophilia is defined as an elevated eosinophil count in the peripheral blood, typically greater than:
- > 500 eosinophils/μL: mild eosinophilia
- > 1,500/μL: marked or hypereosinophilia (may lead to end-organ damage)
Eosinophils are granulocytes involved in immune responses, particularly against parasites, and play a central role in allergic and inflammatory diseases.
Clinical Significance of Eosinophilia
| Eosinophil Level | Clinical Concern |
|---|---|
| 500–1,500/μL | Usually benign, seen in allergies or infections |
| >1,500/μL (persistent) | Risk of tissue injury – check heart, lungs, CNS |
| >5,000/μL | Often seen in parasitic infestations or HES |
Organ damage from eosinophilic infiltration can cause:
- Endomyocardial fibrosis
- Eosinophilic pneumonia
- Neurologic dysfunction
- Skin rashes
Mnemonic: ALLERGIC
Each letter of “ALLERGIC” stands for a key category of causes of eosinophilia:
- A – Adrenal insufficiency (Addison’s disease)
- L – Lymphoma (e.g., Hodgkin’s lymphoma)
- L – L-Tryptophan deficiency
- E – Eczema (allergic skin disease)
- R – Respiratory causes (Asthma, Aspergillosis)
- G – Gastroenteritis
- I – Infections (parasites, fungi)
- C – Collagen vascular diseases (e.g., SLE)
Let’s break them down with clinical context, mechanisms, and diagnostic approaches.
A – Adrenal Insufficiency (Addison’s Disease)
In Addison’s disease, low cortisol levels lead to:
- Increased ACTH and CRH, which promotes eosinophil survival
- Eosinophilia is a classic lab finding
Clinical Clues:
- Hyperpigmentation
- Fatigue, hypotension
- Hyponatremia, hyperkalemia
Diagnosis:
- Low morning cortisol
- High ACTH
- Synacthen stimulation test
Eosinophilia in Addison’s is often subtle but may be the first laboratory abnormality picked up.
L – Lymphoma (Hodgkin’s Lymphoma)
Hodgkin’s lymphoma, particularly the mixed cellularity type, is associated with:
- IL-5 mediated eosinophil proliferation
- Paraneoplastic eosinophilia
Clinical Clues:
- Painless lymphadenopathy (cervical)
- B symptoms: Fever, weight loss, night sweats
- Mediastinal mass
Diagnosis:
- Lymph node biopsy (Reed-Sternberg cells)
- Elevated ESR, eosinophils
- PET-CT staging
Other malignancies associated: T-cell lymphoma, eosinophilic leukemia, and certain solid tumors.
L – L-Tryptophan Deficiency
A lesser-known cause but important:
- L-Tryptophan is a precursor for serotonin and niacin
- Contaminated L-tryptophan supplements have caused eosinophilia-myalgia syndrome
Clinical Features:
- Myalgia
- Skin thickening
- Peripheral eosinophilia
- Possible pulmonary hypertension
This condition highlights how dietary supplements can have serious systemic consequences.
E – Eczema and Allergic Skin Diseases
Eosinophils are major players in Type I hypersensitivity reactions.
Conditions include:
- Atopic dermatitis
- Allergic contact dermatitis
- Pemphigus
- Urticaria
Clues:
- Chronic itching, rashes
- Elevated IgE levels
- Often co-exists with asthma or allergic rhinitis
Investigations:
- Eosinophil count
- Serum IgE
- Skin prick testing or patch testing
Eosinophils are seen in skin biopsy and peripheral smear in moderate to severe cases.
R – Respiratory Causes (Asthma, Allergic Bronchopulmonary Aspergillosis)
Asthma is a classical cause of eosinophilia due to:
- IL-5 mediated recruitment to lungs
- Chronic airway inflammation
ABPA is a severe form of pulmonary hypersensitivity to Aspergillus fumigatus, seen in asthmatics or CF patients.
Clinical Features:
- Recurrent wheezing
- Cough with mucus plugs
- Fleeting pulmonary infiltrates on imaging
Diagnostic Workup:
- Serum IgE > 1000 IU/ml
- Positive Aspergillus-specific IgE
- Eosinophil count >500/μL
Treatment includes corticosteroids and antifungals.
G – Gastroenteritis
Eosinophilic gastroenteritis is a rare but important cause of:
- Abdominal pain
- Diarrhea
- GI bleeding
Eosinophilic infiltration can affect any layer of the bowel:
- Mucosal (diarrhea)
- Muscular (obstruction)
- Serosal (ascites)
Also, parasitic infections causing GI symptoms are under this category.
Diagnosis:
- Peripheral eosinophilia
- Endoscopic biopsy with eosinophil-rich infiltrate
- Rule out other infections
I – Infections (Parasitic and Fungal)
Parasites are the most common cause of eosinophilia globally.
Helminths:
- Strongyloides stercoralis
- Ascaris lumbricoides
- Schistosoma
- Trichinella spiralis
Fungi:
- Coccidioidomycosis
- Histoplasmosis
Eosinophilia typically peaks during larval migration phases (e.g., Loeffler’s syndrome: eosinophilic pneumonia from parasites).
Diagnosis:
- Stool examination
- Serological tests (ELISA)
- Imaging (e.g., cysts in brain, lungs)
C – Collagen Vascular Diseases
Autoimmune conditions often lead to eosinophil recruitment due to chronic inflammation and immune dysregulation.
Includes:
- SLE
- Churg-Strauss Syndrome (EGPA)
- Dermatomyositis
- Scleroderma
Churg-Strauss (Eosinophilic Granulomatosis with Polyangiitis):
- Asthma
- Eosinophilia
- Vasculitis (palpable purpura)
- Mononeuritis multiplex
Investigations:
- ANCA (p-ANCA)
- Eosinophil count
- Skin/nerve/lung biopsy
High-Yield Summary Table: Mnemonic “ALLERGIC”
| Letter | Cause | Clinical Clues | Key Diagnostic Tools |
|---|---|---|---|
| A | Adrenal insufficiency | Hypotension, hyperpigmentation | Cortisol, ACTH, Synacthen test |
| L | Lymphoma (Hodgkin’s) | Lymphadenopathy, night sweats | LN biopsy, PET-CT, CBC |
| L | L-Tryptophan deficiency | Myalgia, rash, supplement history | Exclusion, supplement recall history |
| E | Eczema, Atopic dermatitis | Pruritus, chronic rashes | IgE levels, skin testing |
| R | Respiratory (Asthma, ABPA) | Wheeze, productive cough | IgE, eosinophils, CXR/CT, sputum culture |
| G | Gastroenteritis | Abdominal pain, diarrhea, malabsorption | Endoscopy + biopsy, stool tests |
| I | Infections (Parasites, fungi) | Travel history, GI/lung symptoms | Stool ova, serology, imaging |
| C | Collagen vascular diseases | Systemic symptoms, vasculitis, rash | ANA, ANCA, biopsy |
Investigating Eosinophilia: Diagnostic Algorithm
1. Rule out common causes first:
- Drug allergies
- Asthma
- Parasites (stool exam x 3)
2. Check systemic involvement:
- Heart (echocardiography)
- Lungs (CXR/CT)
- Skin, joints, GI, CNS
- Bone marrow biopsy
- Serum tryptase (for mastocytosis)
- ANCA, ANA (autoimmune diseases)
When is Eosinophilia Dangerous?
Eosinophil levels >1,500/μL for over 6 months, with signs of end-organ damage, point to hypereosinophilic syndrome (HES).
Organs commonly affected:
- Heart (endomyocardial fibrosis)
- Lungs (eosinophilic pneumonia)
- CNS (stroke, confusion)
- GI (eosinophilic colitis)
Treatment: Corticosteroids, anti-IL5 agents (mepolizumab), hydroxyurea in neoplastic variants.
Treatment Approach
| Underlying Cause | Management Strategy |
|---|---|
| Allergies / Asthma | Inhaled or oral corticosteroids |
| Parasitic infections | Albendazole, Ivermectin, Praziquantel |
| Fungal infections | Amphotericin B, Itraconazole |
| Lymphoma | Chemotherapy (e.g., ABVD for Hodgkin's) |
| Autoimmune diseases | Immunosuppressants, steroids |
| Hypereosinophilic syndrome | Corticosteroids, imatinib, biologics |
Frequently Asked Questions (FAQs)
Q1. What is the most common cause of eosinophilia worldwide?
A: Parasitic infections, especially in tropical countries.
Q2. Can drugs cause eosinophilia?
A: Yes. Common culprits: antibiotics (penicillin), anti-TB drugs, NSAIDs, and antiepileptics (carbamazepine).
Q3. What is the role of eosinophils in allergies?
A: Eosinophils release inflammatory mediators (e.g., MBP, ECP) that cause tissue damage in allergic reactions like asthma and dermatitis.
Q4. Is eosinophilia always pathological?
A: No. Mild eosinophilia can occur in healthy individuals, especially post-exercise or during menstruation.
Q5. What is eosinophilic leukemia?
A: A myeloproliferative neoplasm where eosinophils proliferate clonally. Diagnosed via bone marrow biopsy and cytogenetics (e.g., FIP1L1-PDGFRA).
