Hypersensitivity pneumonitis
Lung inflammation from inhaled antigens in sensitized individuals.
Hypersensitivity pneumonitis (HP), also called extrinsic allergic alveolitis (EAA), is a condition that develops when a susceptible or sensitized person repeatedly breathes in environmental antigens. These triggers often include molds, bacteria, bird droppings and feathers, agricultural dusts, bioaerosols, and chemicals found in paints or plastics. The inhaled particles set off a hypersensitivity immune response, leading to inflammation in the lung’s air sacs (alveoli) and small airways (bronchioles). Over time, this inflammation can progress to interstitial lung disease.
**Signs and symptoms** HP is classified as acute, subacute, or chronic, depending on how long the illness lasts.
**Acute** In the acute form, antigen exposure is typically very high but brief. Symptoms usually appear 4 to 6 hours after heavy exposure to the provoking antigen. These include fever, chills, malaise, cough, chest tightness, shortness of breath, rash, swelling, and headache. Once exposure stops, symptoms generally resolve within 12 hours to several days.
**Subacute** People with subacute HP gradually develop a productive cough, shortness of breath, fatigue, loss of appetite, weight loss, and pleuritic chest pain. The symptoms resemble those of the acute form but are less intense and last longer. This pattern may be seen in individuals who have had repeated acute attacks.
**Chronic** In chronic HP, antigen exposure is typically low in volume but continues over a long period. Patients often have no history of acute episodes. The onset is insidious, with cough, progressive shortness of breath, fatigue, and weight loss. The condition is partially to completely reversible, but only slowly, and avoiding further exposure is recommended. Clubbing occurs in about half of patients. Tachypnea, respiratory distress, and inspiratory crackles over the lower lung fields are common. Chronic HP can resemble idiopathic pulmonary fibrosis (IPF) in terms of lung scarring, and many patients experience low blood oxygen levels.
**Epidemiology** The prevalence of HP is thought to be low, but exact numbers are hard to pin down due to diagnostic difficulties, undetected subclinical cases, and variations in climate, region, and proximity to local industries. The most common types are bird fancier’s lung and farmer’s lung. Interestingly, cigarette smoking appears to have a protective effect against th
- field
- Pulmonology, Immunology
- known_for
- Hypersensitivity pneumonitis (extrinsic allergic alveolitis)
- common_types
- Bird fancier's lung, farmer's lung
- prevalence_note
- 0.4–7% of farming population; 20–20,000 cases per 100,000 bird fanciers
Lore & Background
Hypersensitivity pneumonitis can be categorized as acute, subacute, or chronic based on the duration of illness. The acute form follows heavy, short-term antigen exposure, with symptoms developing 4–6 hours later and resolving within 12 hours to several days after exposure ceases. Subacute HP involves gradual onset of productive cough, dyspnea, fatigue, anorexia, weight loss, and pleurisy, with less severe but longer-lasting symptoms. Chronic HP results from low-volume, long-duration antigen exposure, often without acute episodes, and includes insidious cough, progressive dyspnea, fatigue, weight loss, and clubbing in 50% of patients.
Reader's Guide
Hypersensitivity pneumonitis is significant as a preventable interstitial lung disease triggered by environmental and occupational antigens. Diagnosis relies on clinical judgment combining exposure history, imaging, histopathology, pulmonary function testing, serology, and bronchoscopy, with official 2020 guidelines from the American Thoracic Society, Japanese Respiratory Society, and Asociación Latinoamericana del Tórax using high-resolution computed tomography. The disease demonstrates variable prevalence due to diagnostic difficulty and subclinical presentations, with cigarette smoking appearing protective. Understanding HP's pathophysiology—involving type III and type IV hypersensitivity and the two-hit hypothesis of genetic susceptibility plus antigen exposure—guides management, primarily antigen avoidance. Its legacy includes highlighting the importance of occupational and environmental history in respiratory disease.
Did You Know?
- Cigarette smoking appears to be protective against hypersensitivity pneumonitis.
- Only 60% of inciting antigens are identified in exposure assessment.
- The 'head cheese sign' on CT shows a region of lung with three or more different types of attenuation, typical for fibrotic HP.
- Chronic HP can resemble idiopathic pulmonary fibrosis in terms of pulmonary fibrosis and hypoxemia.
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