***Results from bronchoconstriction which results in airway obstruction - this constriction is thought to arise from [[Mast Cells|mast cell]] degranulation and histamine release
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*Histologically
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**[[Bronchi and Bronchioles Inflammatory - Pathology#Acute bronchitis/bronchiolitis|Acute bronchiolitis]], characteristic of the bronchiolar response is the formation of syncytial giant cells (formed by proliferating bronchiolar epithelial cells which may contain [[Degenerations and Infiltrations - Pathology#Cellular Inclusions|intracytoplasmic inclusion bodies]]), alveolar epithelium sometimes affected
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**Obstruction of bronchioles by exudate - these may later become obliterated by the fibrous tissue of organisation
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*May contribute to [[Respiratory Bacterial Infections - Pathology#Enzootic pneumonia of calves|Enzootic pneumonia of calves]]