HP, previously known as “extrinsic allergic alveolitis” (a misnomer, since it not only involves the alveoli but the bronchi as well), is an inflammatory and/or fibrotic  interstitial lung disease characterized by a complex immunological response of the lung parenchyma to repeated inhalation of a sensitizing allergen in susceptible individuals [1].

More than 300 antigens (including bacteria, fungi, animal or plant proteins, metals, low molecular weight chemicals, etc.) have been described to cause HP [1]. However, in 30 to 50% of the cases, it is not possible to identify an inciting antigen [2].

Historically, HP was classified as acute, sub-acute, or chronic based on symptom duration [3]. However, this classification is now regarded as having limited clinical utility due to their lack of correlation with patient outcomes [3]. According to the latest diagnostic guidelines, HP is categorized into non-fibrotic HP (purely inflammatory) and fibrotic HP (mixed inflammatory and fibrotic or purely fibrotic) [2]. Furthermore, when no specific exposure is identified but the clinical presentation aligns with HP, the condition may be referred to as “cryptogenic HP” or “HP of unknown cause” [2]. Fibrotic HP is more severe and has a worse prognosis, with a median survival time ranging between 4.9 and 9.2 years [4].

Although the diagnosis of HP necessitates a multidisciplinary discussion based on various other factors (such as exposure history, serum IgG testing, bronchoalveolar lavage [BAL] and, eventually, histopathological findings), HRCT remains a crucial component of the diagnostic evaluation.