Similar to antibodies and T-cell receptors, Ig-type lectins mediate glycan recognition via an immunoglobulin (Ig)-like domain. The major homologous subfamily of Ig-type lectins are called Siglecs (sialic acid (Sia)-recognizing Ig-superfamily lectins). The siglecs can be broadly classified into two subgroups: Siglecs-1, -2, and -4, and a Siglec-3/CD33-related subgroup (Siglecs-3, and -5 through -13 in primates) defined by sequence similarity and clustered gene localization. They are widely expressed on hematopoietic cells, often in a cell-type-specific manner. Siglec-4/MAG is a myelin component in Schwann cells and oligodendrocytes. Their ligands, sialic acids, are negatively charged monosaccharides found on cell-surface glycoproteins and glycolipids. Although siglec functions continue to be defined, most have intracellular immunoreceptor tyrosine-based inhibitory motifs (ITIM), implicating them in the suppression of immunoreceptor signaling. Siglecs may also participate in cell/cell interactions or act as receptors for the entry of viral or bacterial pathogens.