<unk>  is caused by an exaggerated immune response<unk> . Type III<unk>  and type IV<unk>  can both occur depending on the cause. In general, acute<unk>  is suspected to be attributed to a type III<unk>  while the subacute and chronic types are suspected to be caused by T cell infiltration and granuloma formation. Because different people react variably to antigen exposure, the exact mechanism is unclear but genetic and host factors are likely at play. The two hit hypothesis is often toted in the literature to explain why some people have a normal reaction to an antigenic exposure without clinical findings while others experience an exaggerated immune response. The "first hit" in the hypothesis is proposed to be genetic susceptibility and surrounding environmental factors and the "second hit" would be the introduction of the antigen into the respiratory system which causes the exaggerated immune response.
Hypersensitivity pneumonitis
Idiopathic pulmonary fibrosis
Chemical pneumonitis
Acute eosinophilic pneumonia
Diffuse panbronchiolitis
Cryptogenic organizing pneumonia
Atypical pneumonia
Eosinophilic pneumonia