SFTPD and interstitial lung disease: A total of 12 studies revealed the relationship between serum SP-D levels and the occurrence of ILD (20–31), five studies investigated the significance of serum SP-D in ILD progression (32–36), nine studies explored the clinical value of SP-D in AE-ILD (37–45), and 17 studies reported the serum SP-D levels in patients with ILD between the survival and death groups (32, 44, 46–60).