MUC5AC and hypersensitivity pneumonitis: KL-6 is a mucin-like high-molecular-weight glycoprotein expressed on type 2 alveolar pneumocytes and bronchiolar epithelial cells, and was shown to be a useful serum or bronchoalveolar lavage fluid (BALF) marker for interstitial lung diseases including idiopathic interstitial pneumonia, pneumonitis related to collagen disease, and hypersensitivity pneumonitis (HP) caused by the repeated inhalation of organic antigens [5,6].