Environmental Allergens

Allergen Parade - Meet the Culprits

• Indoor Allergens:
  • House Dust Mites (HDM): Dermatophagoides spp. (Der p 1, Der f 1 in feces). Thrive in humidity >50%.
  • Pet Dander: Cat (Fel d 1), dog proteins.
  • Cockroach: Bla g 1, Bla g 2 from excreta.
  • Molds: Aspergillus, Penicillium spores.
• Outdoor Allergens:
  • Pollens: Seasonal; Trees (Prosopis), Grasses (Cynodon), Weeds (Parthenium).
  • Molds: Alternaria, Cladosporium spores (damp conditions).

⭐ Parthenium hysterophorus (Congress grass) is a major aeroallergen in India, causing allergic rhinitis, asthma, and contact dermatitis.

Body's Overreaction - Allergy Mechanics

• Type I Hypersensitivity: IgE-mediated, immediate reaction.
• Sensitization:
  • Initial allergen exposure → APCs activate Th2 cells.
  • Th2 cells release IL-4, IL-5, IL-13.
  • IL-4 induces B-cell switch to IgE production.
  • IgE binds FcεRI on mast cells/basophils.
• Re-exposure:
  • Allergen cross-links surface IgE on mast cells.
  • Degranulation releases:
    • Preformed: Histamine, tryptase.
    • Newly synthesized: Leukotrienes (LTC4, LTD4, LTE4), Prostaglandins (PGD2).
  • Leads to acute allergic symptoms.
• Late-Phase Reaction (4-12 hrs): Eosinophil, neutrophil, Th2 cell influx; sustained inflammation.

⭐ Mast cell tryptase is a specific marker for mast cell activation and anaphylaxis.

Symptom Symphony - Allergy's Calling Cards

Environmental allergens provoke diverse reactions across organ systems.

• Respiratory Tract:
  • Upper: Allergic rhinitis (sneezing, rhinorrhea, nasal pruritus, congestion), allergic conjunctivitis (itchy, watery, red eyes).
  • Lower: Cough, wheezing, shortness of breath (asthma triggers).
• Skin Manifestations:
  • Urticaria (hives), angioedema.
  • Atopic dermatitis (eczema): pruritic, erythematous rash exacerbations.
• Systemic Reactions:
  • Anaphylaxis: severe, multi-system, potentially fatal.
  • Fatigue, malaise.

⭐ Allergic shiners (infraorbital darkening) & Dennie-Morgan lines (infraorbital creases) are classic signs of chronic allergic states, especially in children.

Detective Work - Pinpointing Allergens

• Goal: Identify specific environmental triggers.
• Initial Approach: Detailed history, physical exam.
• Diagnostic Toolkit:
  • Skin Prick Test (SPT):
    • Rapid, sensitive, cost-effective first-line.
    • Wheal ≥ 3 mm > negative control = positive.
  • Specific IgE (sIgE) Blood Test:
    • Measures allergen-specific IgE.
    • Use if SPT risky/unreliable (e.g., skin disease, antihistamines).
  • Provocation Tests (Nasal, Bronchial):
    • Confirmatory "gold standard"; specialist use.
• Diagnostic Pathway:

⭐ SPT is the primary screening tool for aeroallergens; results in 15-20 mins.

Calming the Storm - Treatment & Prevention

• Avoidance Strategies: Primary prevention.
  • Dust Mites: Encase bedding, wash linens weekly at >55°C, HEPA filters, ↓humidity.
  • Pollen: Close windows during peak season, use AC, HEPA filters.
  • Molds: Control dampness, improve ventilation, use bleach for cleaning.
  • Animal Dander: Remove pet if possible, restrict pet access, HEPA filters.
• Pharmacotherapy: For symptom control.
  • Antihistamines (H1 blockers): Cetirizine, Loratadine.
  • Intranasal Corticosteroids: Fluticasone, Budesonide. 📌 Most effective for allergic rhinitis.
  • Leukotriene Modifiers: Montelukast.
• Allergen Immunotherapy (AIT):
  • SCIT (Subcutaneous) or SLIT (Sublingual).
  • Modifies immune response; offers long-term relief.

⭐ Intranasal corticosteroids are first-line therapy for persistent allergic rhinitis due to their high efficacy and safety profile.

High‑Yield Points - ⚡ Biggest Takeaways

• Key aeroallergens: pollen (seasonal), house dust mites (perennial), fungi, animal dander.
• Mechanism: Predominantly Type I hypersensitivity via IgE and mast cell degranulation.
• Allergic rhinitis: Characterized by sneezing, rhinorrhea, nasal pruritus; note allergic shiners/crease.
• Major triggers for extrinsic (allergic) asthma exacerbations.
• Can exacerbate skin conditions like atopic dermatitis (eczema) and urticaria.
• Diagnosis: Skin prick testing (SPT) and serum allergen-specific IgE are crucial.
• Pillars of management: Avoidance, pharmacotherapy (e.g., antihistamines, steroids), allergen immunotherapy (AIT).