Learning Objectives
Master the classification and pathophysiology of Hypersensitivity Pneumonitis (HP). Understand the combined Type III and Type IV hypersensitivity mechanisms, identify key occupational triggers, and distinguish between acute and chronic presentations for the USMLE Step 1.
1. Pathophysiology and Mechanisms
Hypersensitivity pneumonitis is an immune-mediated inflammatory disease of the lung parenchyma. Unlike asthma, which affects the airways (bronchi), HP primarily affects the alveoli and interstitium.
| Hypersensitivity Type | Mechanism |
|---|---|
| Type III | Immune complex deposition in the alveoli following antigen exposure. |
| Type IV | Delayed-type hypersensitivity involves T-cells and macrophage activation, leading to granuloma formation. |
2. Common Triggers and Occupations
Exposure to specific organic dusts or environmental antigens triggers the immune response.
| Disease Name | Antigen / Source | Scenario |
|---|---|---|
| Farmer’s Lung | Thermophilic Actinomyces | Exposure to moldy hay or grain. |
| Bird-Fancier’s Lung | Avian proteins | Exposure to bird droppings or feathers. |
| Humidifier Lung | Aspergillus/bacteria | Contaminated air conditioning or heating systems. |
3. Acute vs. Chronic Presentation
| Feature | Acute HP | Chronic HP |
|---|---|---|
| Timeline | Hours after heavy exposure. | Months to years of low-level exposure. |
| Symptoms | Dyspnea, cough, fever, headache, chest tightness. | Progressive dyspnea, weight loss, fatigue. |
| Pathology | Infiltrates; often self-limiting if the stimulus is removed. | Irreversible fibrosis, noncaseating granulomas, and traction bronchiectasis. |
| Histology | Alveolar inflammation. | Alveolar septal thickening and fibrosis. |
Activity:
High-Yield Clinical Pearls:
- The “Velcro” Rule: Chronic HP presents with a restrictive PFT pattern (
TLC,
- Granuloma Comparison: Like Sarcoidosis, HP features noncaseating granulomas. However, HP granulomas are usually smaller and localized near the bronchioles.
- Treatment: The most critical step is the strict avoidance of the offending antigen. Steroids may be used for acute flares.