CD33 is an adhesion molecule of myelomonocytic-derived cells that mediates sialic-acid dependent binding to cells. It preferentially binds to alpha-2,6-linked sialic acid. In the immune response, CD33 acts as an inhibitory receptor upon ligand-induced tyrosine phosphorylation. It recruits cytoplasmic phosphatase(s) via their SH2 domain(s), which block signal transduction through dephosphorylation of signaling molecules. CD33 induces apoptosis in acute myeloid leukemia in vitro. In immunohistochemistry, CD33 has membranous and cytoplasmic positivity in populations of hematopoietic and lymphoid cells in immune tissues, the lung, and blood, and it is found on microglial cells in the brain and at low levels in other tissues throughout the body. References: The UniProt Consortium. Nucleic Acids Res. 47: D506-515 (2019); Nucleic Acids Res. 2016 Jan 4;44(D1):D733-45, PMID:26553804;