CD33 or Siglec-3 is a transmembrane receptor expressed on cells of myeloid lineage. It is usually considered myeloid-specific, but it can also be found on some lymphoid cells.
It binds sialic acids, therefore is a member of the SIGLEC family of lectins.
The extracellular portion of this receptor contains two immunoglobulin domains (one IgV and one IgC2 domain), placing CD33 within the immunoglobulin superfamily. The intracellular portion of CD33 contains immunoreceptor tyrosine-based inhibitory motifs (ITIMs) that are implicated in inhibition of cellular activity.
CD33 was the target of gemtuzumab ozogamicin, a monoclonal antibody-based treatment for acute myeloid leukemia, (treatment voluntarily withdrawn from the market in 2010). However, Anti-CD33 monoclonal antibodies are extensively used for the diagnosis of all types of AMLs.