Sialic acid immunoglobulin-like lectin (Siglec) molecules are type I transmembrane proteins with homologous extracellular domains. These molecules consist of inhibitory receptors with immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in the intracellular region and activating receptors that bind to adapter molecules with immunoreceptor tyrosine-based activation motifs (ITAMs). Each Siglec molecule, except for a few, is mainly expressed on immune cells and positively or negatively regulates the activity of the expressing cells by recognizing sialic acid on ligand molecules. It is known that the recognition by each Siglec molecule differs depending on whether the sialic acid on the glycan of the ligand molecules is linked via an α2–3, α2–6, or α2–8 linkage. Furthermore, there are Siglec-like molecules such as paired immunoglobulin-like type 2 receptor α (PILRα) that have a similar three-dimensional structure to Siglec molecules and recognize sialic acid binding to O-type glycans that bind to short-chain peptides with specific sequences.