Hypersensitivity pneumonitis Home A-Z Health Information Health Library A-Z Hypersensitivity Pneumonitis Overview Hypersensitivity pneumonitis (HP), also known as extrinsic allergic alveolitis, is an inflammatory lung disease that is caused by repeated inhalation of organic dusts or other antigens to which an individual is sensitised. These antigens can include mould spores, bacteria, animal proteins, or chemicals found in workplaces or home environments. Causes and Risk Factors HP develops when the immune system exhibits an exaggerated response to these inhaled antigens. Initially, exposure leads to an immune reaction in the small airways and air sacs (alveoli) of the lungs, causing inflammation. With repeated exposure over time, chronic inflammation can lead to scarring of the lung tissue. Symptoms Symptoms of HP can vary depending on the frequency and intensity of antigen exposure. Common symptoms include dry cough, shortness of breath, fatigue and weight loss and flu-like symptoms, particularly after exposure to the antigen. Diagnosis Diagnosis involves a combination of clinical evaluation, imaging-based techniques (e.g., chest X-ray or high-resolution CT scan), pulmonary function tests (PFTs), and sometimes bronchoscopy with bronchoalveolar lavage (BAL) and lung biopsy to confirm the presence of inflammation and rule out other lung diseases. Treatment Treatment includes avoiding further exposure to the offending antigen, which is crucial in managing symptoms and preventing disease progression. In some cases, corticosteroids may be prescribed to reduce inflammation. Severe cases may require immunosuppressive therapy, and in rare instances, lung transplantation.