Ref:Harsh Mohan - Textbook of Pathology, 6th Edition.page no.493 Hypersensitivity (Allergic) Pneumonitis Hypersensitivity pneumonitis is a group of immunologicallymediated ILDs occurring in workers inhaling a variety of organic (biologic) antigenic materials. The condition may have an acute onset due to isolated exposure or may be chronic due to repeated low-dose exposure. ETIOPATHOGENESIS. A list of impoant organic (biologic) dusts which may be inhaled to produce hypersensitivity pneumonitis is already given in. The immunologic mechanisms underlying hypersensitivity pneumonitis from any of these causes appear to be either type III immune-complex disease or type IV delayedhypersensitivity reaction. 1. Farmers' lung is the classic example resulting from exposure to thermophilic actinomycetes generated by humid and warm mouldy hay. 2. Bagassosis occurs in individuals engaged in manufacture of paper and cardboard from sugarcane bagasse. Spores of thermophilic actinomycetes grow rapidly in mouldy sugarcane bagasse which are inhaled. 3. Byssinosis is an occupational lung disease occurring in workers exposed to fibres of cotton, flex and hemp for a number of years. The role of immunologic mechanisms in byssinosis is not as clear as in exposure to other organic dusts. 4. Bird-breeders' (Bird-fanciers') lung occurs in pigeon breeders, parrot breeders, chicken farmers and bird-fanciers who are exposed to bird-droppings and danders from their feathers. 5. Mushroom-workers' lung is found in mushroom cultivators exposed to mushroom compost dust. 6. Malt-workers' lung is seen in distillery and brewery workers who are exposed to mouldy barley and malt dust. 7. Maple-bark disease occurs in those involved in stripping of maple bark and inhale mouldy maple bark (maple tree is grown in nohern hemisphere for timber and its leaf is the national emblem of Canada). 8. Silo-fillers' disease occurs in individuals who enter the silo (silo is an aiight store-house of fodder for farm animals) in which toxic fumes of nitric oxide and nitrogen dioxide are formed due to fermentation of silage. The condition is generally rapidly fatal; less often it may lead to ILD.