– Pulmonary surfactant-associated protein B, also known as SFTPB and SP-B, contains one saposin A-type domain and three saposin B-type domains. SP-B is produced primarily by alveolar type II cells (AEC2) but also by nonciliated respiratory epithelial cells lining distal portions of the respiratory tract. Its secretion promotes alveolar homeostasis, stabilizing lipid layers and lowering surface tension at the air-liquid interface in the peripheral air spaces. Alveolar SP-B influences surfactant formation, effector cell functions, and innate host defense. Deficiency is associated with respiratory distress syndrome (RDS), pulmonary surfactant metabolism dysfunction 1 (SMDP1), and other human lung diseases. Gene addition and editing therapies show promise by complementing SP-B expression in AEC2s, restoring the phenotypic defect in vitro and in vivo.