Cluster of differentiation

Cluster of Differentiation (CD) molecules are markers on the cell surface, as recognized by specific sets of antibodies, used to identify the cell type, stage of differentiation and activity state of a cell.

The CD nomenclature was proposed and established in the 1st International Workshop and Conference on Human Leukocyte Differentiation Antigens (HLDA), which was held in Paris, France in 1982. This system was intended for the classification of the many monoclonal antibodies, generated by different laboratories around the world, against various surface molecules (antigens) on leukocytes (white blood cells). Since then, the use has expanded to other cell types, and more than 150 CD clusters and subclusters have been identified.

CD molecules are not merely markers on the cell surface. Not every CD molecule has been thoroughly characterised, but most of them bring important features to the cells that carry them. For example,

CD31 = PECAM-1, a cell adhesion molecule on platelets and endothelial cells

CD71 = Transferrin receptor, mediates cellular uptake of iron

CD117 = c-kit, the receptor for Stem Cell Factor, a glycoprotein that regulates cellular differentiation, particularly in hematopoiesis

CD120 = a receptor for Tumour Necrosis Factor, an inflammatory cytokine

CD142 = Tissue Factor, a major initiator of blood-clotting

CD143 = Angiotensin-converting enzyme

CD144 = VE-Cadherin, a calcium-dependent adhesion molecule at intercellular junctions, found only in the vascular endothelium

CD202a = Tie2, the receptor for Angiopoietins, a family of angiogenic factors

CD molecules are often referred to when sorting cells by flow cytometry. A '+' or a '–' symbol is used to indicate if a certain fraction of cells possesses or lacks a CD molecule, e.g. "CD34+, CD31–" = expressing CD34, but not CD31.

The most commonly referred to CD molecules are CD4 and CD8, which are markers for two different sub-types of T-lymphocytes. The relative abundance of CD4+ and CD8+ cells reflects the state of one's immune system, and the CD4/CD8 ratio is often used to monitor the progression of an HIV infection.

External links

• HLDA8 (The 8th International Workshop and Conference on Human Leukocyte Differentiation Antigens) to be held in Adelaide, Australia in December, 2004
• CD Index at Protein Reviews On the Web (PROW), NCBI
• A list of CD molecules, at ebioscience.com
• Another list of CD molecules, at Expasy.org
• Yet another list of CD molecules, at PathologyOutlines.com