What is the significance of eosinophils?

Significance of Eosinophils in Clinical Medicine

Eosinophils are multifunctional immune cells that play crucial roles in parasitic infections, allergic disorders, and various inflammatory conditions, serving as both protective immune mediators and potential contributors to tissue damage in pathological states. 1

Key Roles of Eosinophils

Physiological Functions

• Immune Defense: Primary role in protection against parasitic infections, particularly helminth infections 2
• Immune Regulation: Act as modulators of immune homeostasis and can suppress over-reactive pro-inflammatory responses 2
• Tissue Homeostasis: Participate in developmental biology and tissue remodeling processes 3
• Immune Bridging: Connect innate and adaptive immune responses through antigen presentation and interaction with T cells 4

Pathological Significance

• Allergic Disorders: Major contributors to the pathophysiology of:

  • Asthma (airway inflammation) 1
  • Allergic rhinitis
  • Atopic dermatitis
  • Eosinophilic esophagitis (EoE) 5

• Eosinophilic Disorders:

  • Eosinophilic granulomatosis with polyangiitis (EGPA) - characterized by asthma, eosinophilia, and vasculitis 5
  • Hypereosinophilic syndromes
  • Eosinophilic gastrointestinal disorders

Diagnostic Value of Eosinophils

Blood Eosinophilia

• Normal Range: Typically <500 cells/μL
• Peripheral Eosinophilia:
  • Provides supportive evidence for conditions like EoE but is not diagnostic on its own 5
  • Present in 10-50% of adults and 20-100% of children with EoE 5
  • Correlates with disease severity in some cases of EoE 5

Tissue Eosinophilia

• Diagnostic Criteria:
  • In EoE: ≥15 eosinophils per high-power field in esophageal biopsies 6
  • Complete remission defined as <5 eosinophils per 0.3 mm² 6
  • Partial remission defined as 5-15 eosinophils per 0.3 mm² 6

Biomarkers Associated with Eosinophils

• Eosinophil-derived neurotoxin (EDN): Elevated in plasma of EoE patients 5
• Combined biomarkers: When EDN and peripheral eosinophil counts are used together, they provide sensitivity of 63%, specificity of 92%, positive predictive value of 83%, and negative predictive value of 79% 5

Clinical Patterns and Associations

ANCA Status and Eosinophilic Conditions

• In EGPA, ANCA-positive patients (typically anti-MPO) show more vasculitic features including:
  • Glomerulonephritis
  • Peripheral neuropathy
  • Purpura 5
• ANCA-negative EGPA patients more commonly present with:
  • Cardiac involvement
  • Eosinophilic gastroenteritis 5

Allergic Associations

• 50-80% of EoE patients have atopic conditions (allergic rhinitis, asthma, atopic dermatitis) 5
• Blood eosinophilia in EoE often correlates with concurrent allergic sensitization 5
• Seasonal variations in symptoms and esophageal eosinophil levels have been documented in some EoE patients 5

Monitoring Considerations

Follow-up Testing

• Regular monitoring of blood eosinophil counts recommended every 1-3 months in patients with eosinophilic disorders 6
• Blood eosinophil levels should be drawn at diagnosis and at each evaluation for response to treatment 5
• Notation should be made regarding control of concurrent atopic conditions when evaluating eosinophil counts 5

Treatment Response Indicators

• Decreased blood eosinophil counts following therapy (e.g., 88% of patients show decreased counts after fluticasone treatment for EoE) 5
• Histological remission in tissue biopsies 6

Therapeutic Implications

• Targeted Therapies: Eosinophils are important therapeutic targets for allergic diseases 7
• Biologics: Several agents target specific steps in eosinophil differentiation, migration, and activation:
  • Anti-IL-5 agents (mepolizumab, reslizumab)
  • Anti-IL-4/IL-13 (dupilumab)
  • Other biologics in development 6, 7

Pitfalls and Caveats

• Blood eosinophilia alone is not diagnostic of any specific condition and requires clinical correlation
• Normal eosinophil counts do not exclude eosinophilic tissue disorders
• Consider concurrent atopic conditions when interpreting eosinophil counts 5
• When reporting peripheral eosinophilia in research, absolute eosinophil counts and defining criteria should be clearly stated 5
• Vigilance for end-organ damage, particularly cardiac complications, is essential in patients with persistent eosinophilia 6