ABSTRACT

Hypersensitivity pneumonitis (HP) is a lung disease caused by an immune response to inhaled antigen to which the subject has previously been sensitized. HP is essentially defined by the clinical presentation, causative agent, and pathophysiology because the disease has no pathognomonic marker. There is a wide spectrum of clinical presentations from the most acute phase characterized by fever, chills, general malaise, cough, and dyspnea to a more insidious progressive dyspnea over several months if not years. This wide spectrum of presentations has led to the separation of HP into three classical phases: acute, subacute, and chronic. These distinctions are poorly defined and therefore have little clinical relevance. What is important is whether the disease is present or not.