Chapter 42. Hypersensitivity Pneumonitis (Extrinsic Allergic Alveolitis)

Clayton T Cowl, MD, MS, FCCP
DOI: 10.1378/pulm.26.42
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  • Define hypersensitivity pneumonitis and describe examples of environmental sources of antigens associated with this condition.

  • Recognize the complexity of the epidemiology of the disease and associated risk factors.

  • Outline a clinical approach for recognizing the clinical presentation of this condition, including classification of severity of illness and radiographic and histologic features used to ascertain a diagnosis.

  • Discuss treatment strategies for hypersensitivity pneumonitis.

  • Describe clinical syndromes often confused with hypersensitivity pneumonitis but that are considered distinct entities.

  • Review clinical syndromes associated with certain indoor air environments.


Ambient air may contain a variety of irritants, allergens, or toxins that result in dysfunction of the respiratory system. Hypersensitivity pneumonitis involves inhalation of particulate matter, typically organic materials that serve as an antigenic stimulus of an immunologic sensitivity reaction that develops in the alveoli, terminal bronchioles, and interstitium. There is a higher prevalence of nonsmokers who develop this illness, and manifestations of hypersensitivity pneumonitis may be acute, subacute, or chronic. Radiographic findings are often variable but tend to demonstrate poorly defined nodular opacities with variations of ground-glass densities or consolidation most prominent in the lower lobes. Chronic forms typically involve diffuse linear and nodular opacities with an upper-lobe predominance. Laboratory findings include peripheral blood leukocytosis with a predominance of neutrophils and an absence of eosinophils. Bronchoalveolar lavage sampling often reveals lymphocytosis with a low CD4:CD8 ratio. Management centers on removal from the presumed source of inhalation, and judicious use of corticosteroids. There are several other syndromes with similar features that may cause confusion and are more clearly outlined in this chapter. Finally, nonspecific upper respiratory symptoms and headaches have been reported from certain cohorts affected by building-related illnesses. Most of these cases have no identifiable etiology, but seem to involve poor indoor air quality resulting from specific contaminants and more commonly, lack of recirculating fresh air from the ventilation system within the structure itself.


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