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Extrinsic allergic alveolitis

Reviewers: Elliot Weisenberg, M.D. (see Reviewers page)
Revised: 26 September 2011, last major update September 2011
Copyright: (c) 2003-2011, PathologyOutlines.com, Inc.


● Also called hypersensitivity pneumonitis
● Note: many agents cause disease, only most important are discussed
● Spectrum of immunologically mediated, interstitial lung disorders caused by intense, often prolonged exposure to inhaled organic dusts and occupational antigens
● Individuals have abnormal sensitivity or heightened reactivity to these antigens
● Similar to asthma, but involves alveoli, not bronchi
● Acute attacks occur 4-6 hours after inhalation; diffuse/nodular pulmonary infiltrates appear on chest Xray; chronic exposure causes shortness of breath, respiratory failure, cyanosis, decreased total lung capacity and compliance
● Chronic low levels of exposure may cause diffuse interstitial lung disease with fibrosis and honeycomb lung
● Good prognosis, responds well to steroids and withdrawal of offending antigen
● May progress to chronic obstructive lung disease
● Highest risk from dust due to carding and blending; less risk with less dusty operations such as weaving and spinning
● In non-smoking workers, changes mostly reversible upon cessation of dust exposure
Causes: spores of thermophilic bacteria, fungi, animal proteins, bacteria from hay, grain, sugar cane, bark, cheese, cork and animal feces; exposure results in early formation of type III immune complexes, then type IV delayed hypersensitivity
Air conditioner lung: due to thermophilic bacteria
Byssinosis: in textile workers due to fibers from cotton, linen and hemp; resembles asthma clinically; disease mechanism may not be immune mediated, endotoxin from bacterial contamination of cotton may play a role
Farmerís lung: from moldy hay containing spores of thermophilic actinomycetes
Maple bark stripperís lung: fungal spores
Pigeon breederís lung: also called bird fancierís disease; proteins from serum, feathers and excreta


● Remove environmental agent early

Micro description

● Interstitial bronchiolocentric pneumonitis with lymphocytes, plasma cells and foamy macrophages also in alveolar space and terminal airways
● Interstitial fibrosis, obliterative bronchiolitis, poorly circumscribed epithelioid granulomas and intra-alveolar exudate
● Also nodules of organizing fibroblasts, histiocytes and other inflammatory cells
● In byssinosis, hemosiderin coated strands of fiber within fibrous tissue known as byssinoisis bodies may be encountered

Micro images

Giant cells

End of Lung-nontumor > Pneumoconiosis > Extrinsic allergic alveolitis

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