What are the two most common forms of hypersensitivity pneumonitis?

The two most common forms of hypersensitivity pneumonitis are indeed referred to as farmer's lung and bird fancier's lung.

Farmer's lung is associated with the inhalation of moldy hay or straw, which typically contains thermophilic actinomycetes that can trigger an immune response in susceptible individuals. This condition is prevalent among those who are regularly exposed to agricultural dust containing organic material. Bird fancier's lung, on the other hand, is linked to exposure to organic particles and proteins found in bird droppings or feathers, commonly affecting individuals who keep pet birds or work with birds professionally.

Both conditions exemplify the classic mechanisms of hypersensitivity pneumonitis, where an immune-mediated response is triggered by inhaling organic antigens, leading to inflammation in the lungs. Understanding these specific types is essential for diagnosing and managing hypersensitivity pneumonitis effectively, as they represent the most significant sources of exposure for affected populations.