Surface-active protein D (SP-D) has a molecular weight of 43 kDa and is a member of the collagen lectin family with innate immunomodulatory properties. It is constitutively secreted by alveolar lining cells and epithelial cells associated with tubular structures and is induced in cardiac smooth muscle and endothelial cells. Human SP-D is present in serum, plasma, bronchoalveolar lavage fluid, and amniotic fluid.SP-D plays an important role in organismal defense and innate immunity.SP-D binds to neutrophil defensins and regulates antiviral defense. Current studies have shown that SP-D is involved in the development and progression of a variety of lung diseases and may serve as a biomarker for these diseases.