MUC1 and hypersensitivity pneumonitis: Moreover, serum concentrations of KL-6, a molecule elevated in serum of patients with interstitial lung diseases (ILDs), such as idiopathic pulmonary fibrosis and hypersensitivity pneumonitis, was recently proposed to be capable of differentiating between severe and mild COVID-19 patients, being mainly produced by damaged or regenerating alveolar type II pneumocytes [35,36].