Surfactant protein D contributes to the lung's defense against inhaled microorganisms. It's a calcium ion dependent carbohydrate binding protein and is structurally similar to other C type mammalian lectins, such as conglutinin and SPA. It has a molecular size of approximately 37-43kDa in its reduced state, 620kDa in non dissociating conditions. Surfactant protein D enhances the production of oxygen radicals by rat alveolar macrophages and regulates some actions of SPA, which is the most abundant surfactant protein. Surfactant protein D is synthesized and secreted by alveolar epithelial type II cells.