Sialic-acid-binding immunoglobulin-like lectin (Siglec) that plays a role in mediating cell-cell interactions and in maintaining immune cells in a resting state (By similarity). Preferentially binds sialic acid to the short O-linked glycans of certain mucins (PubMed:12773563). Homodimer; disulfide-linked. Interacts with PTPN6/SHP-1 and PTPN11/SHP-2 upon phosphorylation. Interacts with C1QA (via C-terminus); this interaction activates CD33 inhibitory motifs. Expressed on myeloid precursors in the bone marrow. In the peripheral blood, mostly expressed on granulocytes. Glycosylated. Phosphorylation is involved in binding to PTPN6 and PTPN11. CD33-deficient mice are viable and fertile in a pathogen-free environment without any obvious deficiency in overall organ development and growth. Belongs to the immunoglobulin superfamily. SIGLEC (sialic acid binding Ig-like lectin) family. Siglec-3