HP also commonly named extrinsic allergic alveolitis. HP basically is a respiratory syndrome involving the lung parenchyma and specifically the alveoli, terminal bronchiole, and alveolar interstitium, due to a delayed allergic reaction.

The prevalence of HP is a big problem as it is very hard to evaluate because of uncertainties in detection and misdiagnosis and lacking widely accepted diagnostic criteria, and varies considerably depending on disease definition, diagnostic methods, exposure modalities, geographical conditions, agricultural and industrial practices, and host risk factors.

HP can result in exposure to multiple agents that are present in workplaces and in the home, such as microbes, animal and plant proteins, organic and inorganic chemicals. Various environments, settings, and causative agents are increasing over time. HP can be classified into acute/subacute and chronic. This division can be depending on the intensity and frequency of exposure to causative antigens. The mainstay in managing HP is the avoidance of the causative antigen but its avoidance may lead to major changes in lifestyle or occupational settings. HP is a complex syndrome that needs urgently more stringent and selective diagnostic criteria to support the patients.

Reference: https://clinicalmolecularallergy.biomedcentral.com/articles/10.1186/s12948-017-0062-7