SFTPD and chronic obstructive pulmonary disease: SP-D, along with many other candidates like interleukin-6 (IL-6), club cell 16 (CC16), serum soluble receptor for advanced glycation end products (sRAGE), C reactive protein (CRP), and fibrinogen, has been studied to a great extent in COPD patients for disease presentation, progression, and treatment results (Stockley et al., 2019).