[Hypersensitivity Pneumonitis (extrinsic allergic alveolitis)].

Farmer's lung was first described in 1932. We can define hypersensitivity pneumonitis as a pulmonary and systemic disease that is accompanied by dyspnoea and coughing; it is caused by an immunological type of inflammation of the alveolar walls and the terminal airways and it is secondary to the repeated inhalation of a variety of antigens by a susceptible host. It can be said that it is an underdiagnosed disease and only a high degree of clinical manifestations and a detailed history of exposure can lead to an early diagnosis and satisfactory treatment. A combination among clinical-radiological, functional, cytological or pathological findings leads in some cases to a diagnosis. Treatment is based on avoiding further exposure to the causal agent and in the more serious cases the administration of systemic corticoid treatment.