The monoclonal antibody IIIH3 recognizes the rat surfactant protein D (SP-D). SP-D belongs to the collectin familiy. These proteins are oligomeric proteins composed of carbohydrate-recognition domains (CRD) attached to collagenous regions. They are structurally similar to the ficolins although they make use of different CRD structures: C-type lectin domain for the collectins. The anti-microbial effector mechanisms of SP-D are direct opsonization, neutralization, and agglutination. Thus limiting the infection and concurrently orchestrating the subsequent adaptive immune response. 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 gastrointeststinal tract and in epithelial cells of exocrine glands. SP-D synthesis and secretion increase significantly after inflammatory stress. Increased amounts of SP-D in lavage and tissue, particularly in type II pneumocytes, in Clara cells and in hyperplastic goblet cells are found in inflamed lungs. The localization of SP-D in endocytic vesicles and in lysosomal granules of alveolar macrophages suggests that a receptor-mediated uptake occurs. SP-D binds to apoptotic neutrophils and enhances their clearance by alveolar macrophages. Monoclonal antibody IIIH3 specific for rat surfactant protein D shows significant cross reactivity with human SP-D.