Hypersensitivity pneumonitis is a complex disease that has elements of an immunological and cell-mediated allergic response that develops in response to exposure to antigens produced by some species of thermophilic actino-mycetes, such as Micropolyspora faeni, Aspergillus, and other common fungi found as contaminants of grain or hay (73-75).

Ongoing exposure to antigen in sensitized individuals may lead to either a production of antigen-antibody complex (suggesting a type III reaction), or a late-phase cell-mediated response with granuloma formation compatible with a type IV reaction. The immunological response in HP requires prior sensitization and involves recruitment and activation of alveolar neutrophils and macrophages and T-lymphocyte cells. It appears that genetic and environmental interaction plays an important role in HP.

Risk factors for HP appear to be a -308 polymorphism of the tumor necrosis factor-a (TNF-a) promoter gene and polymorphisms of the major histocompatibility complex (74).

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