Hypersensitivity Pneumonitis Serology

Hypersensitivity pneumonitis (HP) is an interstitial lung disease caused by an immunological reaction of the lung parenchyma to repetitive inhalation of an allergen. Both the alveoli and the bronchi become inflamed. Approximately 1 in 100,000 people in the US have HP.

More than 300 antigens have been discovered to cause HP.  Many different names have been given to describe this syndrome depending on the nature of the exposure: 

• Farmer's lung: Seen in agricultural workers involved mostly in livestock farming.
• Bird or Pigeon fancier’s lung: Caused by exposure to organic antigens in bird excreta. Indirect exposure from feather bedding or down comforters have also been reported to cause disease.
• Silo-filler’s disease
• Hot tub lung: Mycobacterium avium complex (MAC) in immunocompetent individuals
• Maple bark-stripper’s disease
• Paprika-slicer’s lung
• Cheese workers lung
• Bagassosis - sugarcane processing
• Mushroom pickers lung
• Malt workers lung

Acute HP typically presents with fever, malaise, cough, and dyspnea within a few hours of heavy exposure to a specific antigen. Symptoms usually resolve within 1 to 2 days of avoiding exposure. Patients also report shortness of breath, malaise, weight loss. The physical examination is often normal, but inspiratory crackles or inspiratory squeaks that reflect small airway inflammation can be heard on pulmonary examination.

Prolonged exposure to the sensitized antigen results in subacute and chronic disease. In chronic HP, crackles may be more prominent, and clubbing may be seen.

Complete blood counts and chemistry panels are usually normal. Inflammatory markers such as erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) are typically elevated.

Serological testing supports the clinical diagnosis of hypersensitivity pneumonitis by detecting IgG antibodies to a number of different environmental antigens. A commonly used testing platform is Phadia ImmunoCap. Patient sera is most commonly tested against a sonicate of antigens derived from Alternaria tenius/aternata, Aspergillus fumigatus, Aureobasidium pullulans, Laceyella sacchari, Micropolyspora faeni, Penicillium chrysogenum/notatum, Phoma beta, and Tricoderma viride.  

Antibody levels greater than the reference range indicate that the patient has been immunologically sensitized to the antigen. A positive IgG antibody only indicates exposure but, by itself, does not confirm diagnosis. For example, 85% of farmers have antibodies and no evidence of disease. The significance of an elevated IgG depends on the patient’s clinical history and other diagnostic tests. 

Specimen requirement is on red top of tube of blood.

References

Morell F, Ojanguren I, Cruz MJ. Diagnosis of occupational hypersensitivity pneumonitis. Curr Opin Allergy Clin Immunol. 2019 Apr;19(2):105-110

Greenberger PA. Hypersensitivity pneumonitis: A fibrosing alveolitis produced by inhalation of diverse antigens. J Allergy Clin Immunol. 2019 Apr;143(4):1295-1301.

Dabiri M et al, Hypersensitivity Pneumonitis: A Pictorial Review Based on the New ATS/JRS/ALAT Clinical Practice Guideline for Radiologists and Pulmonologists, Diagnostics 2022;12(11):2874