CD33 or Siglec-3 is a transmembrane receptor expressed on cells of myeloid

Myeloid

The term myeloid suggests an origin in the bone marrow or spinal cord, or a resemblance to the marrow or spinal cord.In hematopoiesis, the term "myeloid cell" is used to describe any leukocyte that is not a lymphocyte...

 lineage. It is usually considered myeloid-specific, but it can also be found on some lymphoid

Lymphoid

Lymphoid is a term used to describe lymph or the lymphatic system.In the context of lymphoid leukemia, it refers specifically to lymphocytes Lymphoid leukemias and lymphomas are now considered to be tumors of the same type of cell lineage. They are called "leukemia" when in the blood or marrow and...

 cells.

It binds sialic acids, therefore is a member of the SIGLEC

SIGLEC

Siglecs, short for sialic acid binding Ig-like lectins are cell surface receptors and members of the immunoglobulin superfamily that recognize sugars. Their ability to recognize carbohydrates using an immunoglobulin domain places them in the group of I-type lectins...

 family of lectin

Lectin

Lectins are sugar-binding proteins that are highly specific for their sugar moieties. They play a role in biological recognition phenomena involving cells and proteins. For example, some viruses use lectins to attach themselves to the cells of the host organism during infection...

s.

Structure

The extracellular portion of this receptor contains two immunoglobulin domain

Immunoglobulin domain

The immunoglobulin domain is a type of protein domain that consists of a 2-layer sandwich of between 7 and 9 antiparallel β-strands arranged in two β-sheets with a Greek key topology....

s (one IgV and one IgC2 domain), placing CD33 within the immunoglobulin superfamily

Immunoglobulin superfamily

The immunoglobulin superfamily is a large group of cell surface and soluble proteins that are involved in the recognition, binding, or adhesion processes of cells. Molecules are categorized as members of this superfamily based on shared structural features with immunoglobulins ; they all possess a...

. The intracellular portion of CD33 contains immunoreceptor tyrosine-based inhibitory motif

Immunoreceptor tyrosine-based inhibitory motif

An immunoreceptor tyrosine-based inhibition motif , is a conserved sequence of amino acids that is found in the cytoplasmic tails of many inhibitory receptors of the immune system...

s (ITIMs) that are implicated in inhibition of cellular activity.

Clinical significance

CD33 is the target of gemtuzumab ozogamicin

Gemtuzumab ozogamicin

Gemtuzumab ozogamicin is a drug-linked monoclonal antibody that was used to treat acute myelogenous leukemia from 2000-2010...

, a monoclonal antibody-based treatment for acute myeloid leukemia

Acute myeloid leukemia

Acute myeloid leukemia , also known as acute myelogenous leukemia, is a cancer of the myeloid line of blood cells, characterized by the rapid growth of abnormal white blood cells that accumulate in the bone marrow and interfere with the production of normal blood cells. AML is the most common acute...

.