Hypersensitivity pneumonitis (HP), historically referred to as extrinsic allergic alveolitis, is a complex, immune-mediated interstitial lung disease characterized by inflammation of the lung parenchyma, terminal bronchioles, and alveoli resulting from repeated inhalation and subsequent sensitization to a diverse array of environmental or occupational antigens 1, 2. The disease manifests in individuals with underlying immunological susceptibility, wherein exposure to inciting agents—ranging from avian proteins and thermophilic bacteria to various chemical haptens—provokes an aberrant host immune response 5, 6. Clinically and pathologically, hypersensitivity pneumonitis is broadly categorized into nonfibrotic (formerly acute/subacute) and fibrotic phenotypes, each possessing distinct radiologic appearances, histopathologic features, and long-term prognostic implications 2, 3. Because its clinical presentation frequently mimics other interstitial lung disorders, accurate diagnosis requires a rigorous, multidisciplinary approach integrating comprehensive exposure assessment, high-resolution computed tomography (HRCT), bronchoalveolar lavage (BAL) cellular analysis, and, when necessary, surgical lung biopsy 2, 4, 7.
Etiology & Risk Factors
- Avian and Bird Antigens: Exposure to serum, feathers, droppings, or bloom from pigeons, budgerigars, parakeets, chickens, ducks, and geese, commonly designated as bird fancier's lung or pigeon breeder's lung 1, 6.
- Microbial and Fungal Agents: Inhalation of thermophilic actinomycetes (e.g., Saccharopolyspora rectivirgula) in moldy hay causing farmer's lung, or exposure to fungi such as Aspergillus species, Penicillium species, and Trichosporon cutaneum (summer-type hypersensitivity pneumonitis associated with domestic wooden living environments) 1, 5.
- Animal Proteins and Miscellaneous Organic Dusts: Handling animal-derived proteins from laboratory animals (rats, mice), fur dust from animal processing, or vegetable dusts encountered in malt processing, coffee bean handling, and cork manufacturing 1, 6.
- Chemical and Industrial Agents: Exposure to low-molecular-weight chemical haptens—most notably diisocyanates utilized in polyurethane foam manufacturing, spray painting, and plastics industries, as well as acid anhydrides and reactive dyes 1, 6.
- Environmental and Domestic Sources: Contaminated indoor heating, ventilation, and air conditioning (HVAC) systems, humidifiers, hot tubs, and indoor water reservoirs harboring microbial flora (humidifier lung or air conditioner lung) 1, 5.
- Host and Genetic Susceptibility: Polymorphisms in human leukocyte antigen (HLA) genes, variations in immune response regulation pathways, and personal attributes such as non-smoking status (paradoxically associated with higher susceptibility due to altered macrophage and T-cell function) 1, 6.
Pathophysiology
The pathogenesis of hypersensitivity pneumonitis unfolds through an interplay of humoral and cell-mediated immune mechanisms following the inhalation of offending antigens 5. Upon initial exposure, small antigenic particles measuring less than 5 micrometers bypass upper respiratory defenses to deposit directly within terminal bronchioles and alveolar spaces 1. In susceptible hosts, these antigens trigger an initial alveolar macrophage activation and local dendritic cell presentation, culminating in sensitization 5. Subsequent re-exposure initiates a combined immune response involving both Type III (immune complex-mediated, involving precipitating serum IgG antibodies) and Type IV (T cell-mediated delayed hypersensitivity) mechanisms 1, 5.
In the nonfibrotic phase, the cellular infiltrate is dominated by activated CD8+ cytotoxic T lymphocytes and CD4+ helper T cells, which release pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-$\alpha$), interferon-gamma (IFN-$\gamma$), and interleukins (IL-2, IL-8, IL-12) 5. This cytokine milieu drives localized alveolar wall inflammation and the classic formation of poorly formed, non-caseating epithelioid granulomas and multinucleated giant cells distributed predominantly in a bronchiolocentric pattern 3, 5. If antigen exposure persists, the inflammatory cascade transitions into a chronic fibrotic phenotype 3, 5. Activated macrophages and damaged alveolar epithelial cells secrete profibrotic mediators, notably transforming growth factor-beta (TGF-$\beta$) and platelet-derived growth factor (PDGF), which stimulate fibroblast proliferation, myofibroblast accumulation, and excessive extracellular matrix deposition 5. This structural remodeling leads to irreversible distortion of lung architecture, peribronchiolar fibrosis, honeycombing, and severe impairment of gas exchange across the alveolar-capillary membrane 2, 5, 7.
Clinical Presentation & Classic Clinical Signs
- History & Cardinal Symptoms: Patients present with considerable clinical heterogeneity depending on the temporal pattern of exposure 6, 7. Acute hypersensitivity pneumonitis typically presents 4 to 8 hours after a heavy antigen exposure with acute onset of dry cough, dyspnea, fever, chills, malaise, and myalgias, mimicking an acute viral or bacterial respiratory infection 1. Subacute and chronic hypersensitivity pneumonitis feature an insidious onset characterized by progressive exertional dyspnea, chronic dry cough, fatigue, anorexia, and progressive unintended weight loss over months to years, often leading to delayed recognition 1, 6.
- Physical Examination & Named Clinical Signs: Auscultation of the thorax characteristically reveals fine, dry, Velcro-like inspiratory crackles (rales), most prominent over the lower and mid lung zones 1. In advanced chronic and fibrotic cases, physical examination may demonstrate digital clubbing, signs of pulmonary hypertension (such as a loud pulmonic component of the second heart sound or parasternal heave), and peripheral cyanosis indicative of chronic hypoxemia 1. Wheezing is occasionally present due to associated bronchiolar inflammation, whereas upper airway findings are generally unremarkable 1, 3.
Diagnostic Workup & Interpretation (Laboratory, Imaging & ECG/Pathology)
| Diagnostic Modality / Test | Expected Clinical & Radiological / Laboratory Finding |
|---|---|
| Gold-Standard Investigation | Multidisciplinary integration of a verified antigen exposure history, HRCT imaging demonstrating centrilobular nodules and mosaic attenuation, BAL lymphocytosis exceeding threshold values, and compatible histopathology showing bronchiolocentric interstitial pneumonia with or without granulomas 2, 4, 7. |
| Laboratory Panel | Complete blood count often shows leukocytosis with a left shift and elevated inflammatory markers (ESR, CRP) during acute exacerbations. Serum precipitin testing (IgG antibodies against specific inciting antigens) supports exposure but lacks high diagnostic specificity on its own 2, 5. |
| Imaging / Radiology (X-ray, US, CT, MRI) | Chest radiography may be normal or show diffuse reticulonodular infiltrates. High-Resolution CT (HRCT) is essential: nonfibrotic HP displays ground-glass opacities, poorly defined centrilobular nodules, and mosaic attenuation with air trapping on expiratory scans; fibrotic HP reveals traction bronchiectasis, volume loss, reticulation, and occasional honeycombing predominantly in upper-to-mid lung distribution 2, 5. |
| ECG / Histopathology / Procedural | Pulmonary function testing (PFT) demonstrates a restrictive ventilatory defect, reduced forced vital capacity (FVC), and markedly diminished diffusing capacity for carbon monoxide (DLCO). Bronchoalveolar lavage (BAL) yields a profound lymphocytic alveolitis (often 30% to 50%+ lymphocytes). Surgical lung biopsy reveals bronchiolocentric interstitial pneumonia, chronic inflammatory cell infiltrates, non-caseating giant cell granulomas, and peribronchiolar metaplasia 2, 3, 5. |
Staging, Classification & Clinical Scoring Systems
| Criterion / Stage / Score Component | Clinical Threshold, Points & Management Implication |
|---|---|
| Phenotypic Classification (ATS/JRS/ALAT) | Classified strictly into Nonfibrotic HP (absence of fibrosis on HRCT and histology; highly reversible, favorable prognosis) and Fibrotic HP (presence of traction bronchiectasis, reticulation, or honeycombing; progressive clinical course, poorer survival requiring immunosuppressive or antifibrotic management) 2, 3, 7. |
| Diagnostic Confidence Scoring (ATS Guidelines) | Calculated via multidisciplinary evaluation based on exposure history, HRCT patterns, BAL lymphocytosis, and histopathology. Confidence levels categorize diagnosis as Definite (greater than 90%), Probable (70-89%), Possible (50-69%), or Alternative Diagnosis (less than 50%), guiding therapeutic intensity and necessity for invasive biopsy 2, 4. |
| Severity Assessment via PFTs | Impairment staged by FVC and DLCO percentages predicted. Mild (FVC $\ge$ 70%), Moderate (FVC 50-69%), and Severe (FVC < 50%). Severe functional impairment or rapid longitudinal decline mandates aggressive systemic immunosuppression or evaluation for lung transplantation 2, 7. |
Differential Diagnosis
| Differential Condition | Distinguishing Clinical, Biochemical & Imaging Features |
|---|---|
| Idiopathic Pulmonary Fibrosis (IPF) | Characterized by progressive fibrotic interstitial pneumonia with a usual interstitial pneumonia (UIP) pattern on HRCT (subpleural, basal predominance with honeycombing) lacking a clear environmental antigen history, and BAL fluid typically lacking marked lymphocytosis 2, 7. |
| Sarcoidosis | Features non-caseating granulomas on biopsy similar to HP, but HRCT classically demonstrates symmetric bilateral hilar lymphadenopathy, perilymphatic nodular distribution, and absence of characteristic mosaic air-trapping or a clear organic antigen exposure history 1, 5. |
| Connective Tissue Disease-Associated ILD (CTD-ILD) | Associated with systemic autoimmune manifestations, positive serologies (ANA, RF, anti-CCP, myositis-specific antibodies), arthralgias, and specific rheumatologic findings rather than environmental antigen inhalation 2, 4. |
| Atypical Pneumonia / Respiratory Infections | Presents with acute fever, cough, and infiltrates, but lacks the chronic exposure history, persistent mosaic attenuation on HRCT, and profound lymphocytic BAL profile characteristic of subacute or chronic hypersensitivity pneumonitis 1, 5. |
Management & Treatment Protocol
- Initial Stabilization & Resuscitation: Immediate removal of the patient from the offending environmental or occupational antigen source is the foundational cornerstone of all management protocols 2, 5, 7. Supplemental oxygen therapy is administered targeting an oxyhemoglobin saturation $\ge$ 92% at rest and during exertion for patients exhibiting hypoxemia 1, 7.
- First-Line Medical / Pharmacological Therapy (with exact adult dosing): For patients with symptomatic nonfibrotic hypersensitivity pneumonitis or acute exacerbations of chronic disease, systemic corticosteroid therapy is initiated 5, 7. Standard induction dosing comprises oral prednisolone at 0.5 mg/kg/day (maximum initial dose of 40 mg daily) for 2 to 4 weeks, followed by a gradual taper over several months based on clinical symptoms, serial PFTs, and HRCT improvement 5, 7. In cases requiring steroid-sparing agents due to toxicity or refractory fibrotic disease, second-line immunosuppressants such as azathioprine (starting at 50 mg daily titrated up to 2 to 2.5 mg/kg/day, maximum 150-200 mg daily) or mycophenolate mofetil (starting at 500 mg twice daily titrated up to 1,000 to 1,500 mg twice daily) are utilized 5, 7. For progressive fibrotic phenotypes despite conventional immunosuppression, antifibrotic therapy with nintedanib (150 mg orally twice daily with food) is indicated to slow the rate of forced vital capacity decline 5, 7.
- Surgical / Interventional Indications: Referral for formal pulmonary rehabilitation programs to improve exercise tolerance and functional capacity 7. Patients with end-stage progressive fibrotic hypersensitivity pneumonitis refractory to maximal medical therapy and antigen avoidance should be evaluated early for single or double lung transplantation evaluation 5, 7.
Complications & Prognosis
- Pulmonary Hypertension & Cor Pulmonale: Chronic hypoxia and progressive loss of the pulmonary capillary bed lead to fixed pulmonary vascular remodeling, severe pulmonary arterial hypertension, right ventricular strain, and secondary right heart failure (cor pulmonale) 1, 5.
- Progressive Respiratory Failure & Fibrosis: Irreversible parenchymal destruction in fibrotic phenotypes results in chronic respiratory failure requiring long-term oxygen therapy, frequent hospitalizations for acute exacerbations, and marked functional limitation 2, 7.
- Prognostic Factors: The clinical trajectory is heavily dictated by the specific phenotype 2, 7. Nonfibrotic hypersensitivity pneumonitis with prompt antigen elimination carries a favorable, often completely reversible prognosis 2, 5. Conversely, fibrotic hypersensitivity pneumonitis is associated with progressive lung function decline, early mortality rates comparable to idiopathic pulmonary fibrosis, and poor long-term survival unless lung transplantation is successfully performed 2, 7.