Surfactant protein D (SP-D) belongs to the collectin family.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 Ⅱcells 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.