hypersensitivity pneumonitis

Hypersensitivity pneumonitis is what happens when the lungs become allergic to something a person repeatedly breathes in. The culprits are usually fine organic particles: mould spores in damp hay, proteins from bird feathers and droppings, or microbes growing in humidifiers and hot tubs. The immune system overreacts to these inhaled antigens and inflames the small airways and alveoli.

It comes in different tempos. An acute, flu-like attack can strike hours after a heavy exposure, with fever, cough, and breathlessness that often improves once the person is away from the trigger. A chronic form develops insidiously with long, lower-level exposure and can lead to permanent fibrosis that resembles other progressive interstitial diseases.

Diagnosis hinges on linking the illness to an exposure, supported by CT findings, blood tests for specific antibodies, bronchoalveolar lavage showing a rise in lymphocytes, and sometimes biopsy. A careful environmental and occupational history is one of the most powerful diagnostic tools, because the exposure may be a hobby, a workplace, or even a home.

The single most effective treatment is removing the offending antigen, whether that means rehoming pet birds, fixing damp, or changing a job. Corticosteroids can calm acute inflammation. Once fibrosis is established, however, antigen avoidance and antifibrotic therapy may slow but not undo the damage.