What is the role of eosinophils in the body and how are conditions with elevated eosinophil counts treated?

The Role of Eosinophils and Management of Eosinophilic Conditions

Eosinophils are pleiotropic multifunctional leukocytes that play critical roles in host defense against parasites, allergic inflammation, and tissue homeostasis, with treatment of eosinophilic conditions primarily involving corticosteroids, targeted biologics, and allergen avoidance depending on the specific disorder. 1, 2

Eosinophil Biology and Normal Function

Structure and Development

• Eosinophils are fully delineated granulocytes that mature in the bone marrow before circulating in peripheral blood
• Development is regulated by transcription factor GATA-1 and interleukin-5 (IL-5) 3
• Normal blood eosinophil count ranges from 0.05-0.5 × 10^9/L (typically <4% of white blood cells) 4

Physiological Functions

• Host defense against parasites: Particularly effective against helminthic infections
• Immune modulation: Interact with mast cells and T cells to regulate both innate and adaptive immunity
• Tissue homeostasis: Contribute to developmental biology and normal tissue maintenance
• Response to infections: Enhance immunity against certain molds and viruses 2, 3

Pathological Role in Disease

Mechanisms of Tissue Damage

• Release of cytotoxic granule proteins (eosinophil-derived neurotoxin, major basic protein)
• Production of reactive oxygen species
• Generation of lipid mediators (leukotrienes)
• Release of proinflammatory cytokines 5, 3

Eosinophilic Disorders

1. Respiratory disorders:

   • Asthma (particularly eosinophilic phenotypes)
   • Chronic rhinosinusitis with nasal polyps
   • Eosinophilic pneumonia
   • Allergic bronchopulmonary aspergillosis 2

2. Gastrointestinal disorders:

   • Eosinophilic esophagitis (EoE)
   • Eosinophilic gastroenteritis 6, 3

3. Systemic disorders:

   • Hypereosinophilic syndrome (HES)
   • Eosinophilic granulomatosis with polyangiitis (EGPA)
   • Drug reactions with eosinophilia and systemic symptoms (DRESS) 4, 1

Diagnostic Approach

Blood Testing

• Complete blood count with differential to confirm eosinophilia (>0.5 × 10^9/L)
• Peripheral blood smear to evaluate blood cell morphology
• IgE levels to assess for allergic etiology 6, 4

Tissue Evaluation

• Biopsy of affected tissues to confirm eosinophilic infiltration
• For EoE: ≥15 intraepithelial eosinophils per high-power field is diagnostic 6

Additional Workup

• Detailed clinical history, including:
  • Travel to tropical/subtropical areas
  • Exposure to potential allergens
  • Medication history
  • Family history of allergic disorders 4

Treatment Strategies

Targeting Underlying Causes

1. For parasitic infections:

   • Albendazole 400 mg daily for 3 days for hookworm (90% cure rate)
   • Ivermectin 200 μg/kg for strongyloidiasis (90% cure rate)
   • Specific antiparasitic regimens based on identified organism 4

2. For allergic disorders:

   • Allergen avoidance when triggers are identified
   • Topical corticosteroids (e.g., fluticasone 440-880 mg twice daily for adults with EoE)
   • Dietary elimination in food-triggered disorders 6, 4

Pharmacological Management

1. First-line therapy:

   • Corticosteroids (systemic or topical depending on condition)
   • For idiopathic hypereosinophilic syndrome: prednisolone 30-60 mg/day 4

2. Biologic therapies:

   • Anti-IL-5 agents (mepolizumab, reslizumab)
   • Anti-IL-5 receptor antibodies (benralizumab)
   • Anti-IgE therapy (omalizumab) for allergic conditions 6, 4

3. Immunomodulatory treatments:

   • For refractory cases: hydroxyurea, interferon-α
   • IVIG for selected cases of hyper IgE syndrome 4

Advanced Interventions

• Hematopoietic stem cell transplantation for severe hyper IgE syndrome
• Aggressive prophylactic antibiotic therapy for recurrent infections 4

Monitoring and Follow-up

• Regular monitoring of eosinophil counts (every 1-3 months)
• Assessment for end-organ damage, particularly cardiac complications
• Vigilance for disease recurrence after treatment discontinuation 4

Special Considerations

• Peripheral eosinophilia correlates with disease severity in many conditions but is not diagnostic alone
• In eosinophilic esophagitis, 10-50% of adults and 20-100% of children have elevated peripheral eosinophil counts 6
• Blood eosinophil levels >0.24 × 10^9/L predict tissue eosinophilia with 70.9% sensitivity and 78.4% specificity 6

By understanding the dual role of eosinophils in health and disease, clinicians can better diagnose and manage eosinophilic conditions, ultimately improving patient outcomes through targeted therapeutic interventions.