20th February 2025

Overview of HP

Hypersensitivity pneumonitis (HP) is characterised by granulomatous lung inflammation due to various inhaled organic antigens. It involves immune complex and cell-mediated (types III and IV hypersensitivity reactions) tissue damage.

Exposure to organic antigens may result from various hobbies or occupations, which often provide the name for different subtypes of HP:

• Farmer’s lung – due to exposure to Saccharopolyspora rectivirgula in mouldy hay
• Bird-fancier’s lung – due to exposure to avian proteins on bird feathers and droppings
• Cheese-worker’s lung – due to exposure to Penicillium casei in cheese mould
• Malt-worker’s lung – due to exposure to Aspergillus clavatus in mouldy malt
• Mushroom-worker’s lung – due to exposure to thermophilic actinomycetes in mushroom compost
• Chemical-worker’s lung – due to exposure to trimellitic anhydride, diisocyanate and methylene diisocyanate in the manufacture of plastics, polyurethane foam and rubber
• Hot-tub lung – due to exposure to Mycobacterium avium in hot tubs

Traditionally, HP is divided into acute and chronic forms. The acute form, which develops after a short period of exposure to the antigen, causes pneumonitis. The chronic form, which requires longer exposure to the causative organism, tends to cause fibrosis and emphysema.