SFTPD and interstitial lung disease: The levels of Krebs von den Lungen-6 (KL-6) [2,21], surfactant protein-D (SP-D) [2,21], α-defensin [35], and periostin [36,37] in the blood further increase in AE-IPF compared to those in the chronic phase, which are produced from damaged alveolar epithelial cells, and changes in KL-6 levels [38] or periostin [37] were significant prognostic factors in patients with AE-ILD.