A 35-year-old man with a 10-year history of asthma presents with recurrent episodes of fever, eosinophilia (absolute count 4500/μL), and pulmonary infiltrates on chest X-ray that migrate from one lobe to another over weeks. He denies recent infections or drug use. Serum IgE level is markedly elevated at 2800 IU/mL. Sputum examination shows Aspergillus fumigatus. Which type of hypersensitivity reaction best explains the pathophysiology of his pulmonary disease?

Explanation

## Allergic Bronchopulmonary Aspergillosis (ABPA) This patient presents with **allergic bronchopulmonary aspergillosis (ABPA)**, a complex hypersensitivity disorder involving **both Type I and Type III hypersensitivity reactions**. The predominant pathophysiologic mechanism is **Type III hypersensitivity** (immune complex-mediated disease). ### Clinical Features of ABPA | Feature | Finding in This Case | |---------|----------------------| | **Predisposition** | Pre-existing asthma (10 years) | | **Fungal colonization** | *Aspergillus fumigatus* in sputum | | **Pulmonary findings** | Migratory infiltrates (fleeting opacities) | | **Systemic manifestations** | Fever, malaise | | **Laboratory markers** | Elevated IgE (>1000 IU/mL), eosinophilia | | **Immunologic basis** | IgE + IgG immune complexes | ### Pathophysiologic Mechanisms in ABPA **Key Point:** ABPA involves a dual hypersensitivity response: 1. **Type I Hypersensitivity (IgE-mediated)** - IgE antibodies against *Aspergillus* antigens bind to mast cells - Causes acute bronchoconstriction and bronchial hyperresponsiveness - Contributes to elevated serum IgE 2. **Type III Hypersensitivity (Immune Complex-mediated)** — **PRIMARY MECHANISM** - IgG antibodies against *Aspergillus* antigens form immune complexes - Complexes deposit in pulmonary tissue (bronchial walls, alveoli) - Complement activation (C3, C5) → recruitment of neutrophils and eosinophils - Results in **eosinophilic inflammation**, bronchitis, and bronchiectasis - Causes the characteristic **migratory pulmonary infiltrates** (fleeting opacities) ### Why Type III Hypersensitivity Is Predominant **High-Yield:** The key diagnostic clues pointing to Type III are: - **Migratory infiltrates** — hallmark of immune complex deposition shifting as antigen-antibody ratios change - **Eosinophilia** — complement-mediated recruitment of eosinophils (not typical of pure Type I) - **Systemic symptoms** (fever) — characteristic of immune complex disease - **Elevated IgG antibodies** against *Aspergillus* (Type III mechanism) - **Chronic, progressive course** — Type III reactions are typically delayed and persistent **Clinical Pearl:** ABPA is distinguished from simple allergic asthma (pure Type I) by the presence of: - Pulmonary infiltrates (not just bronchospasm) - Marked eosinophilia - Elevated IgE *and* IgG anti-*Aspergillus* antibodies - Migratory (not fixed) infiltrates ### Pathologic Changes ```mermaid flowchart TD A[Aspergillus fumigatus colonization]:::outcome --> B[IgE + IgG antibody production]:::outcome B --> C{Type of Response}:::decision C -->|IgE-mediated| D[Mast cell degranulation]:::action D --> E[Acute bronchoconstriction]:::outcome C -->|IgG-mediated| F[Immune complex formation]:::action F --> G[Complement activation]:::action G --> H[Neutrophil + Eosinophil infiltration]:::action H --> I[Bronchitis, bronchiectasis, migratory infiltrates]:::outcome E --> J[ABPA phenotype]:::outcome I --> J ``` **Mnemonic:** **ABPA = Asthma + Bronchiectasis + Pulmonary infiltrates + Aspergillus** → Think immune complexes (Type III) + IgE (Type I)