Pulmonary Pathology Online
Pathology of Extrinsic Allergic Alveolitis
Hypersitivity Pneumonitis ; Farmer's Lung
Extrinsic allergic alveolitis is a response to inhaled allergens, usually large proteins.
The prototype is "farmerís lung", caused by the inhalation of thermophilic actinomycetes, notably Saccharopolyspora rectivirgula (formerly Micropolyspora faeni).
These organisms grow in moldy hay.
Classically, a farmworker enters a barn shortly after animals have changed from grazing to eating hay and rapidly develops tightness in the chest , shortness of breath, cough, and mild fever.
The lag period is several hours, so the symptoms may only appear after leaving the barn.
The symptoms remit but return on exposure. However, with time the symptoms become chronic.
More important, in many instances the initial symptoms may not be noticed or may be attributed to a "virus", and the patient may present with slowly progressive dyspnea, as in interstitial pneumonia.
In the rare instances in which tissue has been studied in the acute phase, bronchiolar necrosis, eosinophilic infiltrate, vasculitis, and interstitial pneumonia were observed.
More often patients are seen in the subacute or chronic phase, when the biopsy is characteristic.
An extensive interstitial pneumonia is characterized by a heavy infiltrate of lymphocytes and a few plasma cells in the alveolar walls, an appearance resembling that of lymphocytic interstitial pneumonia.
Mild diffuse alveolar damage is usually present.
Unlike lymphocytic interstitial pneumonia, there is a significant bronchiolar infiltrate, sometimes with bronchiolitis obliterans.
As a consequence focal areas of endogenous lipid pneumonia are seen.
Most characteristic is the presence of scattered, poorly formed granulomas that contain foreign body giant cells, some of which exhibit doubly refractile material or needle-like clefts.
The nature of this material is unknown but is thought to be particles in the dusty hay.
The lung architecture is usually recognizable, but mild fibrosis may occur, usually in the alveolar walls.
In the chronic (end-stage) phase, the interstitial inflammation recedes, but fibrosis is more apparent, the lung architecture is distorted and honeycombing occurs.
Besides material in moldy hay there are more than 20 other substances that produce extrinsic allergic alveolitis.
In pigeon breederís or bird fancierís lung, the antigen in bird protein.
The common bird is the parakeet, but chicken handlers also develop the condition. [Chronic bird fancier's lung (BFL) can be subgrouped into two types.
One subgroup of patients develops interstitial pulmonary fibrosis after recurrent acute episodes (recurrent BFL), and the other subgroup of patients has no history of acute episodes but has slowly progressive chronic respiratory disease (insidious BFL).]
Other rare causes of extrinsic allergic alveolitis include pituitary snuff, fungi in bark (maple bark stripperís lung), cork (suberosis), malt, mushrooms, and detergents.
Ingested drugs, notably methotrexate, also produce a similar lesion.
It is important to realize that there may be no apparent antigen exposure and that extrinsic allergic alveolitis may be caused by fungi growing in stagnant water in air-conditioners and central heating units.
It is essential to distinguish extrinsic allergic alveolitis from usual interstitial pneumonia because removal of the antigen is the only adequate treatment.
The multiplicity of exotic names describing the many different circumstances in which the specific allergens that cause extrinsic allergic alveolitis may be inhaled, is at variance with the single pathological picture, which is characterized by :
- Fairly diffuse interstitial infiltration by lymphocytes and plasma cells, progressing to interstitial fibrosis.
- Scanty poorly formed granulomas or single giant cells (often with an acicular cholesterol crystals cleft), disappearing within 6 months of exposure to the allergen.
- Tendency for the disease to be maximal about the bronchioles.
- Foci of intra-alveolar and bronchiolar fibrosis.
- Except in the acute stages, the disease preponderates in the upper lobes like other "granulomatous" diseases, such as sarcoidosis , eosinophilic granuloma, silicosis, and tuberculosis, in contrast to such "exudative" processes as cryptogenic fibrosing alveolitis and asbestosis.
- Sarcoidosis differs in that the granulomas are well-formed and are set in otherwise normal alveolar tissue (lacking the widespread interstitial pneumonitis seen in extrinsic allergic alveolitis).
- Cryptogenic fibrosing alveolitis differs in affecting the subpleural region of the lung and the periphery of the lobules, lacking the bronchiolocentricity of extrinsic allergic alveolitis.
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