Table 1.

Summary of surfactant proteins.

Proteins	Structural features	Examples of Functions
SP-A (i) a	Octadecameric glycoprotein, 650 kDa [56], acidic	Facilitates clearance of pathogens and immune effectors [57], inhibits secreted phospholipases A2 activity and maintains surfactant integrity during lung injury [58]
SP-B (o) a	79-amino-acid homodimer with disulfide-linked, 18 kDa [59]	Enhances adsorption of phospholipids from subphase to interface [60]. Increases collapse pressure of fatty acids to avoid “squeeze-out”
SP-C (o) a	Nonpolar α-helical protein containing 35 amino acids, 4.2 kDa [61]	Stabilizes phospholipids [62,63], increases viscosity of interfacial film [64]
SP-D (i) a	Glycoprotein, dodecamer of four trimmers, 43 kDa [65]	Regulates surfactant metabolism [66], promotes epithelial cells to uptake pathogenic bacteria [67]

^a“i” represents hydrophilic and “o“represents hydrophobic