What evidence supports the diagnosis of hypersensitivity pneumonitis?

Hypersensitivity pneumonitis, also known as extrinsic allergic alveolitis, is an immunologically mediated inflammatory response in the lungs due to inhalation of organic antigens. A key feature of this condition is the marked lymphocytic response in the bronchoalveolar lavage (BAL) fluid, which serves as strong evidence supporting a diagnosis of hypersensitivity pneumonitis.

In this disorder, the exposure to the offending antigen leads to an accumulation of lymphocytes, particularly CD4+ T-helper cells, in the alveolar spaces. This lymphocytic predominance in BAL fluid is a reflection of the underlying immune-mediated process and differentiates hypersensitivity pneumonitis from other types of lung diseases.

While abnormalities on chest X-rays may be indicative of various lung conditions, they do not uniquely confirm hypersensitivity pneumonitis. Similarly, pulmonary function tests typically reveal restrictive patterns rather than obstruction, which could be due to several other lung disorders. Lastly, positive skin prick tests are generally associated with immediate hypersensitivity reactions, such as allergic rhinitis or asthma, rather than the delayed hypersensitivity and immune reactions that characterize hypersensitivity pneumonitis. Thus, the notable increase in lymphocyte count in BAL fluid is the most compelling and specific evidence for