Hypersensitivity pneumonitis

HP Pathophysiology - The Allergic Lung

• Definition: A diffuse, granulomatous interstitial lung disease (ILD) from repeated inhalation of an organic antigen in a sensitized individual.
• Immunologic Mechanism: A combination of hypersensitivity reactions, distinct from asthma.
  • Type III (Immune Complex): Antigen-antibody complexes deposit in alveoli, activating complement and causing acute inflammation.
  • Type IV (Delayed, Cell-Mediated): T-cell activation leads to the formation of characteristic granulomas upon chronic exposure.

⭐ The presence of poorly formed non-caseating granulomas on biopsy is a classic finding, but not pathognomonic; it's also a key feature of sarcoidosis.

Etiologies & Antigens - Feather & Spore Culprits

Inhaled organic dusts containing antigens are the primary triggers. Diagnosis hinges on identifying the specific exposure. Key syndromes include:

⭐ Farmer's Lung is the most common type of hypersensitivity pneumonitis.

Clinical Picture & Diagnosis - The Telltale Crackles

• Clinical Presentations Vary:
  • Acute: Abrupt onset (4-8 hrs post-exposure) of flu-like symptoms (fever, myalgia), cough, & dyspnea. Resolves with antigen avoidance.
  • Subacute: Insidious onset of productive cough & dyspnea over weeks to months.
  • Chronic: Progressive dyspnea, cough, and fatigue, leading to irreversible pulmonary fibrosis.

⭐ On high-resolution CT (HRCT), the combination of ground-glass opacities, centrilobular nodules, and air trapping (mosaic attenuation) is highly suggestive of HP.

Management & Prognosis - Dodge, Suppress, Survive

• Antigen Avoidance: The absolute cornerstone of management. Complete removal of the offending agent is paramount for halting disease progression.
• Corticosteroids: Mainstay for symptomatic acute or subacute disease (e.g., prednisone) to suppress inflammation.
• Chronic/Fibrotic HP: May require long-term steroids or other immunosuppressants (e.g., mycophenolate, azathioprine). Lung transplantation is the final option for severe, end-stage fibrotic disease.
• Prognosis: Excellent for acute HP if the antigen is removed; guarded to poor for chronic fibrotic HP.

⭐ The single most important factor determining prognosis in hypersensitivity pneumonitis is the identification and removal of the causative antigen.

• A Type III/IV hypersensitivity reaction to inhaled organic antigens (e.g., moldy hay in Farmer's Lung, avian proteins in Bird Fancier's Lung).
• Acute form presents with sudden fever, cough, and dyspnea 4-8 hours post-exposure.
• Chronic exposure leads to insidious progressive dyspnea and irreversible pulmonary fibrosis.
• Imaging shows diffuse ground-glass opacities (acute) or reticular/nodular patterns with honeycombing (chronic).
• BAL reveals marked lymphocytosis.
• Antigen avoidance is the most critical step in management.