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 collagen‐like region, an α‐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 and D, which are secreted into the pulmonary alveolar and airway lining fluid. The collectins represent an important group of pattern recognition molecules, which bind to oligosaccharide structures and/or lipid moities on the surface of microorganisms. They bind preferentially to monosaccharide units of the mannose type, which present two vicinal hydroxyl groups in an equatorial position. High‐affinity interactions between collectins and microorganisms depend, on the one hand, on the high density of the carbohydrate ligands on the microbial surface, and on the other, on the degree of oligomerization of the collectin. Apart from binding to microorganisms, the collectins can interact with receptors on host cells. Binding of collectins to microorganisms may facilitate microbial clearance through aggregation, complement activation, opsonization and activation of phagocytosis, and inhibition of microbial growth. In addition, the collectins can modulate inflammatory and allergic responses, affect apoptotic cell clearance and modulate the adaptive immune system.

Infection with gram-positive bacteria is a major cause of pneumonia. Surfactant proteins A (SP-A) and D (SP-D) are thought to play an important role in the innate immunity of the lung. Both proteins can bind to gram-positive bacteria. Until now, it was not known with which surface component(s) of gram-positive bacteria SP-A and SP-D interact. Lipoteichoic acid (LTA) and peptidoglycan (PepG) are components of the cell wall of gram-positive bacteria. By use of a solid phase-based binding assay, LTA of Bacillus subtilis was shown to be bound by SP-D but not by SP-A. Unmodified PepG of Staphylococcus aureus was bound by SP-D. SP-D binding to both LTA and PepG was calcium dependent and carbohydrate inhibitable. These results indicate that SP-D interacts with gram-positive bacteria via binding to the cell wall components LTA and PepG and that the carbohydrate recognition domain is responsible for this binding.

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Pulmonary surfactant protein (SP)-D is an important component of the innate immune system of the lung, which is thought to function by binding to specific carbohydrates on the surface of viruses and unicellular pathogens. SP-D has been shown to have a relatively high affinity for the monosaccharides mannose, glucose, and fucose. However, there is limited information on SP-D binding to complex carbohydrate structures, and binding of SP-D to fucose in the context of an oligosaccharide has not yet been investigated. In this study, we used surface plasmon resonance spectroscopy to examine the potential of SP-D to bind to various synthetic fucosylated oligosaccharides, and identified Fucalpha1-3GalNAc and Fucalpha1-3GlcNAc elements as strong ligands. These types of fucosylated glycoconjugates are presented at the surface of Schistosoma mansoni, a parasitic worm that, during development, transiently resides in the lung. In line with the findings by surface plasmon resonance, we found that SP-D can bind to larval stages of S. mansoni, demonstrating for the first time that SP-D interacts with multicellular lung pathogens.