CD33 is a type I Lectin belonging to the Ig superfamily. CD33 contains an N terminal Ig like V type domain, which mediates sialic acid binding, followed by one Ig like C2 type domain, a transmembrane region and a cytoplasmic tail containing two conserved immunoreceptor tyrosine based inhibition motifs (ITIMs). Eleven human Siglecs have been characterized. Siglecs 5 to 11 share a high degree of sequence similarity with CD33/Siglec3 both in their extracellular and intracellular regions. They are collectively referred to as CD33 related Siglecs. CD33 related Siglecs have differential expression pattern within the hematopoietic system. They are involved in the regulation of cellular activation within the immune system. Siglec 3 expression is restricted to cells of myelomonocytic lineage. Siglec3 recruits SHP1 and SHP2 to its ITIMs upon phosphorylation.