Surfactant protein D (SP-D) belongs to the collectin familiy. These proteins are oligomeric proteins composed of carbohydrate-recognition domains (CRD) attached to collagenous regions. Collectins are structurally similar to the ficolins although they make use of different CRD structures: C-type lectin domain for the collectins. The lung is the major site of synthesis of SP-D, where the molecules are produced and secreted onto the epithelial surface by alveolar type II cells and unciliated bronchial epithelial cells. SP-D is also found in different epithelial cells of the gastrointestinal tract and in epithelial cells of various exocrine glands. SP-D is an important factor in the pulmonary anti-microbial defense. The anti-microbial defense mechanisms of SP-D are direct opsonization, neutralization and agglutination. This results in limiting the infection and concurrently orchestrating the subsequent adaptive immune response. SP-D synthesis and secretion increase significantly during inflammatory stress. In inflamed lungs increased amounts of SP-D in bronchial lavage fluid and tissue are found, particularly in type II pneumocytes, Clara cells and hyperplastic goblet cells are found in inflamed lungs. The localization of SP-D in endocytic vesicles and in lysosomal granules of alveolar macrophages suggests a receptor-mediated uptake. SP-D is also involved the apoptotic process. SP-D binds to apoptotic neutrophils and enhances their clearance by alveolar macrophages. Although complete deficiency of SP-D has not been described in man, basal levels are highly variable and are largely genetically determined. SP-D levels in healthy individuals vary from 200 ng/ml to 5200 ng/ml. Mean SP-D levels increase from 700 ng/ml in children to 1100 ng/ml in adults. Bronchoalveolar fluid SP-D control levels range from 7 ng/ml to 28 µg/ml. SP-D levels can rise dramatically during acute pulmonary infections and are raised in certain non-infectious lung diseases.The ELISA is not influenced by the presence of lipopolysaccharides.