SIGLEC

Siglecs, short for sialic acid binding Ig-like lectins are cell surface receptors and members of the immunoglobulin superfamily (IgSF) that recognize sugars. Their ability to recognize carbohydrates using an immunoglobulin domain places them in the group of I-type (Ig-type) lectins. They are transmembrane proteins that contain an N-terminal V-like immunoglobulin (IgV) domain that binds sialic acid and a variable number of C2-type Ig (IgC2) domains.

The first described Siglec is sialoadhesin (Siglec-1/CD169) that is a lectin-like adhesion molecule on macrophages. Other Siglecs were later added to this family, including CD22 (Siglec-2), which is restricted to B cells and has an important role in regulating their adhesion and activation, CD33 (Siglec-3) and myelin-associated glycoprotein (MAG/Siglec-4). Several additional Siglecs (Siglecs 5–12) have been identified in humans that are highly similar in structure to CD33 so are collectively referred to as ‘CD33-related Siglecs’. CD33-related siglecs all have two conserved immunoreceptor tyrosine-based inhibitory motif (ITIM)-like motifs in their cytoplasmic tails suggesting their involvement in cellular activation.