Human Siglec-5 are Itype(Igtype) lectins belonging to the Ig superfamily,They are characterized by an N terminal Ig-like V type domain which mediates sialic acid binding, followed by varying numbers of Ig-like C2 type domains. SIGLEC5 has also been designated CD170, they are expressed by monocytic or myeloid lineage cells, and also found at high levels in peripheral blood leukocytes, spleen, bone marrow and at lower levels in lymph node, lung, appendix, placenta, pancreas and thymus. SIGLEC5 are expressed by monocytes and neutrophils but absent from leukemic cell lines representing early stages of myelomonocytic differentiation. Siglec5 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. One remarkable feature of the CD33 related Siglecs is their differential expression pattern within the hematopoietic system This fact, together with the presence of two conserved immunoreceptor tyrosinebased inhibition motifs (ITIMs) in their cytoplasma tails, suggests that CD33 related Siglecs are involved in the regulation of cellular activation within the immune system.