Siglecs and their roles in the immune system

Through binding ubiquitous sialic-acid residues on cell surfaces, the Siglec family of lectins promote cell–cell interactions and regulate the functions of numerous immune-cell types. This Review describes the emerging roles of Siglecs in pathogen recognition and endocytosis. Cell surfaces in the immune system are richly equipped with a complex mixture of glycans, which can be recognized by diverse glycan-binding proteins. The Siglecs are a family of sialic-acid-binding immunoglobulin-like lectins that are thought to promote cell–cell interactions and regulate the functions of cells in the innate and adaptive immune systems through glycan recognition. In this Review, we describe recent studies on signalling mechanisms and discuss the potential role of Siglecs in triggering endocytosis and in pathogen recognition. Finally, we discuss the postulated functions of the recently discovered CD33-related Siglecs and consider the factors that seem to be driving their rapid evolution.