Extrinsic Allergic Alveolitis
Key Facts
Terminology
Hypersensitivity pneumonitis (HP)
Granulomatous lung disease secondary to inhalation of organic antigens or chemicals
Etiology/Pathogenesis
History of exposure to organic antigens
Saccharopolyspora tectivirgula in “farmer’s lung” (moldy hay)
Thermoactinomyces sacchari in bagassosis (sugar cane workers)
Animal protein from bird feathers and feces from birds
Clinical Issues
Acute hypersensitivity pneumonitis
4-6 hours after exposure to antigen
Episode may last 12-18 hours
Chronic hypersensitivity pneumonitis
Patients may have continuous exposure to antigen over longer periods of time
Patients may not experience acute episode
This type of HP eventually gets biopsied
Treatment
Corticosteroids
Top Differential Diagnoses
Sarcoidosis
Usual interstitial pneumonitis (UIP)
BOOP
Diagnostic Checklist
Areas of BOOP
Noncaseous granulomatous inflammation
 Hypersensitivity pneumonitis shows the 2 most important features for its diagnosis: Areas of bronchiolitis obliterans  and the presence of ill-defined granulomas  . |
 In other areas of lung parenchyma, there is a discrete presence of numerous noncaseating granulomas  . The presence of ill-defined granulomas is characteristic of HP. |
TERMINOLOGY
Synonyms
Hypersensitivity pneumonitis (HP)
Definitions
Granulomatous lung disease secondary to inhalation of organic antigens or chemicals
ETIOLOGY/PATHOGENESIS
Environmental Exposure
History of exposure to organic antigens
Saccharopolyspora rectivirgula in “farmer’s lung” (moldy hay)
Thermoactinomyces sacchari in bagassosis (sugar cane workers)
Animal protein from bird feathers and feces from birds
Pathogenesis
HP represents immune complex-mediated (type III) T-cell and delayed (type IV) hypersensitivity reaction
CLINICAL ISSUES
Epidemiology
Incidence
Exact incidence of HP is unknown
However, HP may be more prevalent in people exposed to birds
Age
Usually in adults
Gender
No predilection for any particular gender
Ethnicity
No predilection for any particular ethnic group
Site
Lung without predilection for specific side or lung segment
Presentation
Cough
Dyspnea
Fever
Chills
Wheezing
Malaise
Laboratory Tests
Pulmonary function tests may be abnormal
Erythrosedimentation may be elevated
C-protein may be elevated
Natural History
Acute hypersensitivity pneumonitis
4-6 hours after exposure to antigen
Episode may last 12-18 hours
Chronic hypersensitivity pneumonitis
Patients may have continuous exposure to antigen over months or years
Patients may not experience acute episode
This type eventually gets biopsied
