Surfactant Protein D (SP-D) is a 43 kDa member of the collectin family of innate immune modulators. It is constitutively secreted by alveolar lining cells and epithelium associated with tubular structures and induced in cardiac smooth muscle and endothelial cells. Human SP-D is found in serum, plasma, broncho-alveolar lavage fluid, and amniotic fluid. SP-D plays an important role in host defense and innate immunity. It binds human neutrophil defensins, modulating influenza anti-viral defense. It has increasing evident that SP-D involves in the pathogenesis of many pulmonary diseases, and serves as a biomarker.