Collectins are a family of collagenous calcium-dependent defense lectins in animals. Their polypeptide chains consist of four regions: a cysteine-rich N-terminal domain,a colla-gen-like region,ana-helical coiled-coil neck domain and a C-terminal lectin or carbohydrate-recognition domain. These polypeptide chains form trimers that may assemble into larger oligomers. The best studied family members are the mannan-binding lectin,which is secreted into the blood by the liver,and the surfactant proteins A andD,which are secreted into the pulmonary alveolar and airway lining fluid
