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Extrinsic allergic alveolitis

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Extrinsic allergic alveolitis

Hypersensitivity pneumonitis
Classification and external resources
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This is a high magnification photomicrograph from a lung biopsy taken from a patient with chronic hypersensitivity pneumonitis (hematoxylin-eosin stain). It shows mild expansion of the alveolar septa (interstitium) by lymphocytes. A multinucleated giant cell, seen within the interstitium to the right of the picture halfway down, is an important clue to the correct diagnosis.
ICD-10 9 DiseasesDB MedlinePlus eMedicine MeSH D000542

Hypersensitivity pneumonitis (HP; also called extrinsic allergic alveolitis, EAA) is an inflammation of the alveoli within the lung caused by hypersensitivity to inhaled organic dusts. Sufferers are commonly exposed to the dust by their occupation or hobbies.


Hypersensitivity pneumonitis involves inhalation of an antigen. This leads to an exaggerated immune response (hypersensitivity). Type III hypersensitivity and type IV hypersensitivity occur in hypersensitivity pneumonitis.[1]


Hypersensitivity pneumonitis (HP) is categorized as acute, subacute, and chronic based on the duration of the illness.[2]


In the acute form of HP, symptoms may develop 4–6 hours following heavy exposure to the provoking antigen. Symptoms include fever, chills, malaise, cough, chest tightness, dyspnea, and headache. Symptoms resolve within 12 hours to several days upon cessation of exposure.[3]

Acute HP is characterized by poorly formed noncaseating interstitial granulomas and mononuclear cell infiltration in a peribronchial distribution with prominent giant cells.[3]

On chest radiographs, a diffuse micronodular interstitial pattern (at times with ground-glass density in the lower and middle lung zones) may be observed. Findings are normal in approximately 10% of patients." In high-resolution CT scans, ground-glass opacities or diffusely increased radiodensities are present. Pulmonary function tests show reduced diffusion capacity of lungs for carbon monoxide (DLCO). Many patients have hypoxemia at rest, and all patients desaturate with exercise.[3]


Patients with subacute HP gradually develop a productive cough, dyspnea, fatigue, anorexia, weight loss, and pleurisy. Symptoms are similar to the acute form of the disease, but are less severe and last longer. On chest radiographs, micronodular or reticular opacities are most prominent in mid-to-lower lung zones.[3] Findings may be present in patients who have experienced repeated acute attacks.

The subacute, or intermittent, form produces more well-formed noncaseating granulomas, bronchiolitis with or without organizing pneumonia, and interstitial fibrosis.[3]


In chronic HP, patients often lack a history of acute episodes. They have an insidious onset of cough, progressive dyspnea, fatigue, and weight loss. This is associated with partial to complete but gradual reversibility. Avoiding any further exposure is recommended. Clubbing is observed in 50% of patients. Tachypnea, respiratory distress, and inspiratory crackles over lower lung fields often are present.[3]

On chest radiographs, progressive fibrotic changes with loss of lung volume particularly affect the upper lobes. Nodular or ground-glass opacities are not present. Features of emphysema are found on significant chest films and CT scans.[3]

Chronic forms reveal additional findings of chronic interstitial inflammation and alveolar destruction (honeycombing) associated with dense fibrosis. Cholesterol clefts or asteroid bodies are present within or outside granulomas.[3]

In addition, many patients have hypoxemia at rest, and all patients desaturate with exercise.


The diagnosis is based upon a history of symptoms after exposure to the allergen and clinical tests. A physician may take blood tests, seeking signs of inflammation, a chest X-ray and lung function tests. The sufferer shows a restrictive loss of lung function.

Although overlapping in many cases, hypersensitivity pneumonitis may be distinguished from occupational asthma in that it is not restricted to only occupational exposure, and that asthma generally is classified as a type I hypersensitivity.[4][5] Unlike asthma, hypersensitivity pneumonitis targets lung alveoli rather than bronchi.[6]

Lung biopsy

Lung biopsies can be diagnostic in cases of chronic hypersensitivity pneumonitis, or may help to suggest the diagnosis and trigger or intensify the search for an allergen. The main feature of chronic hypersensitivity pneumonitis on lung biopsies is expansion of the interstitium by lymphocytes accompanied by an occasional multinucleated giant cell or loose granuloma. [7] [8]

When fibrosis develops in chronic hypersensitivity pneumonitis, the differential diagnosis in lung biopsies includes the idiopathic interstitial pneumonias.[9] This group of diseases includes usual interstitial pneumonia, non-specific interstitial pneumonia and cryptogenic organizing pneumonia, among others. [10] [11]

The prognosis of some idiopathic interstitial pneumonias, e.g. idiopathic usual interstitial pneumonia (i.e. idiopathic pulmonary fibrosis), are very poor and the treatments of little help. This contrasts the prognosis (and treatment) for hypersensitivity pneumonitis, which is generally fairly good if the allergen is identified and exposures to it significantly reduced or eliminated. Thus, a lung biopsy, in some cases, may make a decisive difference.


Hypersensitivity pneumonitis may also be called many different names, based on the provoking antigen. These include:

Type[12] Specific antigen Exposure
Bird fancier's lung
Also called bird breeder's lung, pigeon breeder's lung, and poultry worker's lung
Avian proteins Feathers and bird droppings [13]
Bagassosis Thermophilic actinomycetes[13] Moldy bagasse (pressed sugarcane)
Cephalosporium HP Cephalosporium Contaminated basements (from sewage)
Cheese-washer's lung Penicillum casei[13] or P. roqueforti Cheese casings
Chemical worker’s lung - Isocyanate HP Toluene diisocyanate (TDI), Hexamethylene diisocyanate (HDI), or Methylene bisphenyl isocyanate (MDI) Paints, resins, and polyurethane foams
Chemical worker's lung[13] - Trimellitic anhydride (TMA) HP Trimellitic anhydride[13] Plastics, resins, and paints
Coffee worker's lung Coffee bean protein Coffee bean dust
Compost lung Aspergillus Compost
Detergent worker's disease Bacillus subtilis enzymes Detergent
Familial HP
Also called Domestic HP
Bacillus subtilis, puffball spores Contaminated walls
Farmer's lung The molds Moldy hay
Hot tub lung Mycobacterium avium complex Mist from hot tubs
Humidifier lung The bacteria
  • Thermoactinomyces candidus
  • Bacillus subtilis
  • Bacillus cereus, and Klebsiella oxytoca;
  • Thermophilic actinomycetes[13]

the fungi

and the amoebae

  • Naegleria gruberi,
  • Acanthamoeba polyhaga, and
  • Acanthamoeba castellani.
Mist generated by a machine from standing water
Japanese summer house HP Also called Japanese summer-type HP
Trichosporon cutaneum Damp wood and mats
Laboratory worker's lung Male rat urine protein Laboratory rats
Lycoperdonosis Puffball spores Spore dust from mature puffballs[14]
Malt worker's lung Aspergillus clavatus[13] Moldy barley
Maple bark disease Cryptostroma corticale[13] Moldy maple bark
Metalworking fluids HP Nontuberculous mycobacteria Mist from metalworking fluids
Miller's lung Sitophilus granarius (wheat weevil)[13] Dust-contaminated grain[13]
Mollusc shell HP Aquatic animal proteins Mollusc shell dust
Mushroom worker's lung Thermophilic actinomycetes Mushroom compost
Peat moss worker's lung Caused by Monocillium sp. and Penicillium citreonigrum Peat moss
Pituitary snuff taker's lung Pituitary snuff Medication
Sauna worker's lung Aureobasidium, Graphium spp Contaminated sauna water
Sequoiosis Aureobasidium, Graphium spp Redwood bark, sawdust
Streptomyces HP Streptomyces albus Contaminated fertilizer
Suberosis Penicillium glabrum (formerly known as Penicillium frequentans) Moldy cork dust
Tap water HP Unknown Contaminated tap water
Thatched roof disease Saccharomonospora viridis Dried grass
Tobacco worker's lung Aspergillus spp Moldy tobacco
Wine-grower's lung Botrytis cinerea mold Moldy grapes
Woodworker's lung Alternaria, Penicillium spp Wood pulp, dust

Of these types, Farmer's Lung and Bird-Breeder's Lung are the most common. "Studies document 8-540 cases per 100,000 persons per year for farmers and 6000-21,000 cases per 100,000 persons per year for pigeon breeders. High attack rates are documented in sporadic outbreaks. Prevalence varies by region, climate, and farming practices. HP affects 0.4-7% of the farming population. Reported prevalence among bird fanciers is estimated to be 20-20,000 cases per 100,000 persons at risk." [3]


The best treatment is to avoid the provoking allergen, as chronic exposure can cause permanent damage. Corticosteroids such as prednisolone may help to control symptoms but may produce side-effects.[15]

Additional images


  • Oxford Handbook of Clinical Medicine
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