All mammalian cells display a diverse array of glycan structures that differ from those found on microbial pathogens. Siglecs are a family of sialic acid-binding immunoglobulin-like receptors that participate in the discrimination of ‘self’ and ‘non-self’ and regulate the functions of cells in the innate and adaptive immune systems through recognition of their glycan ligands. In this review, we describe the recent advances in our understanding of the roles of Siglecs in the regulation of immune cell functions in infectious diseases, inflammation, neurodegeneration, autoimmune diseases and cancer.

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The structure of the functional N-terminal domain from the extracellular region of the cell surface receptor sialoadhesin has been determined in complex with the oligosaccharide 3' sialyllactose. This provides structural information for the siglec family of proteins. The structure conforms to the V-set immunoglobulin-like fold but contains several distinctive features, including an intra-beta sheet disulphide and a splitting of the standard beta strand G into two shorter strands. These novel features appear important in adapting the V-set fold for sialic acid-mediated recognition. Analysis of the complex with 3'sialyllactose highlights three residues, conserved throughout the siglec family, as key features of the sialic acid-binding template. The complex is representative of the functional recognition interaction with carbohydrate and as such provides detailed information for a heterotypic cell adhesion interaction.