Causes of hypersensitivity pneumonitis
More than two hundred antigens have been associated with hypersensitivity pneumonitis. Common ones include dusts from bird feathers and droppings, moulds found on hay, straw and grain, dampness in the home, metalworking fluids and some medications.
Doctors believe that the immune response occurs as a consequence of repeated, often low-level exposure to these antigens over a period of time.
The immune response causes inflammation of the lungs, particularly the air sacs (alveoli) and smaller airways, making it harder for the lungs to work.
Scientists believe that genetic factors may explain why some people are more likely to develop the disease.
There are two types of hypersensitivity pneumonitis – acute hypersensitivity pneumonitis and chronic hypersensitivity pneumonitis (CHP).
In acute HP, symptoms develop within a few hours after intense or heavy exposure to the antigen. As well as breathlessness and cough, people may experience fever, chest tightness, fatigue and a headache.
In chronic hypersensitivity pneumonitis, symptoms generally develop slowly over time and include breathlessness, cough, fatigue and weight loss.
Hypersensitivity pneumonitis can be difficult to diagnose. The symptoms of acute HP are similar to those of asthma or acute respiratory tract infection whereas those of chronic hypersensitivity pneumonitis may mimic other forms of pulmonary fibrosis, such as idiopathic pulmonary fibrosis (IPF).
Early and accurate diagnosis is important to ensure removal of the source of the dust or allergen (for example removal of birds, wearing filtration face mask at work, changing job).
Acute hypersensitivity pneumonitis is usually treatable if correctly diagnosed and there is no more exposure to the antigen. However, in many cases it cannot be identified.
Chronic hypersensitivity pneumonitis is a long-term condition. You may need to take steroids or other immunosuppressive therapies to reduce the inflammation in your lungs.
Some patients with chronic hypersensitivity pneumonitis may also be considered for lung transplantation.
After your diagnosis, you will have regular follow-up appointments with your medical team to monitor your lung disease.
If acute HP is diagnosed early, it may resolve so long as there is no more exposure to the substance involved. But, in about a third of affected people, the lung disease can progress even when there is no further exposure to the antigen, resulting in scar tissue or fibrosis and irreversible damage to the lungs.
Chronic hypersensitivity pneumonitis may get worse over time in a manner similar to Idiopathic Pulmonary Fibrosis.
Also known as:
Hypersensitivity pneumonitis is also known as extrinsic allergic alveolitis. The most common types of hypersensitivity pneumonitis are:
- Bird keeper’s lung
- Pigeon fancier’s lung
- Chemical worker’s lung
- Metal worker’s lung
- Farmer’s lung
- Hot tub lung
Other forms of pulmonary fibrosis
- COVID-19-related pulmonary fibrosis
- Drug-induced pulmonary fibrosis
- Idiopathic Pulmonary fibrosis
- Rheumatoid arthritis-ILD
- Systemic sclerosis
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