Hypersensitivity Pneumonitis – Lung Allergy to Inhaled Dust

Pneumonitis is the term for inflammation of the lungs, involving the respiratory and often the terminal bronchioles as well. This may arise as a result of trauma (chemical/physical), allergic reactions, airborne particles and drugs. It is often confused with pneumonia – pneumonitis is a broad term that includes lung inflammation from any cause while pneumonia is often reserved for lung inflammation due to an infection. However, the term pneumonia is more popular and to some extent has replaced the word pneumonitis to describe lung inflammation arising from infectious and non-infectious causes. Pneumoconiosis is lung disease caused by the inhalation of mineral (inorganic) dusts and may be seen as a type of pneumonitis.

What is hypersensitivity pneumonitis?

Hypersensitivity pneumonitis is inflammation of the lung as a result of an immune reaction to  inhaled organic airborne particles. When these particles make contact with the walls of the bronchioles and alveoli, the antigen-antibody immune complexes (type III hypersensitivity response) and the involvement of immune cells in response to the antigens (type IV hypersensitivity response) propagates the inflammatory process. Read more on types of allergic reactions. Hypersensitivity pneumonitis is also known as extrinsic allergic alveolitis.

Causes of Hypersensitity Pneumonitis

Hypersensitivity pneumonitis may arise with exposure to organic dusts and particles particularly those containing bacterial and fungal spores, fungi, animal proteins or bacterial toxins. Despite the involvement of microorganisms like bacteria and fungi, hypersensitivity pneumonitis is NOT an infection but an allergic response to the antigens on the microorganism’s surface and its products.

It is seen with a number of occupations and past times, particularly in those working with animals and birds, in agriculture and food processing often dealing with grains.

Types of Hypersensitivity Pneumonitis

The more common types of hypersensitivity pneumonitis includes :

  • Pigeon breeder’s lung, also known as bird breeder’s / bird fancier’s lung due to inhalation of bird feces and feathers (particles). It may also be seen with rodent breeding.
  • Farmer’s lung due to exposure to hay, straw and grain tainted with molds. The major antigen includes Saccharopolyspora rectivirgula and Thermoactinomyces vulgaris.
  • Byssinnosis is seen in textile workers who are exposed to dust from cotton, flax and hemp.

Less common types of hypersensitivity pneumonitis includes :

  • Malt worker’s lung due to inhalation of moldy malt – Aspergillus clavatus.
  • Cheese worker’s lung caused by moldy cheese – Aspergillus clavatus, Penicillium casei.
  • Tobacco worker’s lung caused tobacco molds – Aspergillus species.
  • Air-conditioner lung / humidifier feverSaccharopolyspora rectivirgula and Thermoactinomyces vulgaris.
  • Maple bark stripper’s lungCryptostroma corticale.

Signs and Symptom of Hypersensitivity Pneumonitis

Hypersensitivity pneumonitis can be acute or chronic. The acute stages is marked by exposure to large quantities of the dust in a short period of time – within hours. In the chronic setting, there may be inhalation of small quantities of the organic dust over prolonged periods of times – months or years.

Acute symptoms includes :

  • Difficulty breathing
  • Dry cough
  • Fever and/or chills
  • Headache
  • Muscle ache and joint pain

Chronic hypersensitivity pneumonitis is associated with pulmonary fibrosis (scarring of the lung tissue) which impairs lung functioning. The chronic symptoms may be more subtle and includes :

  • Shortness of breath
  • Persistent cough

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