A 35-year-old farmer from rural Maharashtra develops recurrent episodes of wheezing, nasal congestion, and pruritus 2–4 hours after exposure to stored grain dust. Symptoms resolve within 24 hours of leaving the farm. Serum IgE is markedly elevated (850 IU/mL). Which investigation is most appropriate to confirm occupational allergic sensitization and identify the causative antigen?

Explanation

## Type I Hypersensitivity in Occupational Allergy ### Clinical Presentation This patient has occupational allergic rhinitis and asthma triggered by grain dust — a classic Type I hypersensitivity reaction. The key features are: - **Temporal relationship:** symptoms 2–4 hours after exposure (consistent with Type I) - **Reversibility:** resolution within 24 hours of removal from antigen - **Elevated total IgE:** 850 IU/mL (normal <150 IU/mL) suggests atopic sensitization - **Antigen source:** stored grain dust contains storage mites and their fecal allergens ### Investigation of Choice: Serum-Specific IgE to Storage Mites **Key Point:** Serum-specific IgE measurement identifies the causative allergen in occupational Type I hypersensitivity and confirms IgE-mediated sensitization. **High-Yield:** Storage mites (Dermatophagoides pteronyssinus, D. farinae, Acarus siro, Tyrophagus putrescentis) are ubiquitous in stored grain and are major allergens in agricultural workers. **Mnemonic:** **DAMP** — Dermatophagoides, Acarus, Mites in stored Products ### Why Serum IgE Over Other Tests? | Investigation | Role in Type I | Utility Here | Limitation | | --- | --- | --- | --- | | **Serum-specific IgE** | Gold standard | Identifies mite allergen; confirms IgE sensitization | Moderate cost; 24–48 hr turnaround | | **Skin prick test** | High sensitivity/specificity | Can be done; safe in non-acute phase | Requires allergen extract availability; operator-dependent | | **Patch testing** | Type IV hypersensitivity | Not indicated; patch test detects delayed, not immediate, reactions | Wrong mechanism; will be negative | | **Complement fixation** | Type II/III reactions | No role in Type I; complement not primary mediator | Detects antibody-antigen complexes, not IgE | **Clinical Pearl:** In occupational allergy, serum-specific IgE is preferred over skin testing because it: 1. Identifies the exact allergen (mite species, specific protein) 2. Avoids risk of anaphylaxis in a patient with high baseline IgE 3. Provides objective, quantifiable results 4. Can be performed regardless of skin condition or concurrent medications ### Pathophysiology Storage mites produce proteolytic enzymes in their feces. These allergens are inhaled, processed by antigen-presenting cells, and trigger Th2 differentiation. Th2 cells promote B-cell class switching to IgE. IgE binds mast cells in respiratory mucosa; re-exposure causes cross-linking, degranulation, and release of histamine and leukotrienes → rhinitis, asthma.