The cluster of differentiation (often abbreviated as CD) is a protocol used for the identification and investigation of cell surface molecules present on leukocytes. CD molecules can act in numerous ways, often acting as receptors or ligands (the molecule that activates a receptor) important to the cell. A signal cascade is usually initiated, altering the behavior of the cell (see cell signaling). Some CD proteins do not play a role in cell signaling, but have other functions, such as cell adhesion. There are approximately 250 different proteins.^[1]

Nomenclature

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 in 1982.^[2][3] This system was intended for the classification of the many monoclonal antibodies (mAbs) generated by different laboratories around the world against epitopes on the surface molecules of leukocytes (white blood cells). Since then, its use has expanded to many other cell types, and more than 320 CD unique clusters and subclusters have been identified. The proposed surface molecule is assigned a CD number once two specific monoclonal antibodies (mAb) are shown to bind to the molecule. If the molecule has not been well-characterized, or has only one mAb, it is usually given the provisional indicator "w" (as in "CDw186").

Uses as cell markers

CD differentiation. (Captions in German)

CD molecules are utilized in cell-sorting using various methods including flow cytometry. Cell populations are usually defined using a '+' or a '–' symbol to indicate whether a certain cell fraction expresses or lacks a CD molecule. For example, a "CD34+, CD31–" cell is one that expresses CD34, but not CD31. This CD combination typically corresponds to a stem cell, opposed to a fully-differentiated endothelial cell.

|- 
| stem cells 

|- 
| all leukocyte groups 

|- 
| Granulocyte  

|- 
| Monocyte 

|- 
| T lymphocyte 

|- 
| T helper cell 

|- 
| Cytotoxic T cell 

|- 
| B lymphocyte 

|- 
| Thrombocyte 

|- 
| Natural killer cell 

Two commonly-used CD molecules are CD4 and CD8, which are, in general, used as markers for helper and cytotoxic T cells, respectively. When defining T cells, these molecules are defined in combination with CD3+, as some other leukocytes also express these CD molecules (some macrophages express low levels of CD4; dendritic cells express high levels of CD8). CD4 is also an essential receptor during HIV infection, allowing both the binding of HIV to helper T cells and the destruction of CD4+ T cells. The relative abundance of CD4+ and CD8+ T cells is often used to monitor the progression of an HIV infection.

Other uses

It is important to note that, while CD molecules are very useful in defining leukocytes, they are not merely markers on the cell surface. While only a fraction of known CD molecules have been thoroughly characterised, most of them have an important function. In the example of CD4 & CD8, these molecules are critical in antigen recognition.

See also

• List of human clusters of differentiation for a list of CD molecules
• Immune system
• Immune tolerance
• Antibodies
• Leukocytes
• Signal transduction

References

External links

• CD list maintained by the Human Cell Differentiation Molecules Council (successor to the HLDA Workshops)
• CD Antibody Review
• A list of CD molecules, at ebioscience.com
• Another list of CD molecules, at Expasy.org
• Another list of CD molecules, at Sciencegateway.org
• Yet another list of CD molecules, at PathologyOutlines.com
• Catalogue of some CD-factors, as R&D Systems.
• Wall charts of CD molecules and other cytokines, with colors, arrows from one cell to another, from eBioscience. Was supplement to Science 313(5792) 8 September 2006.

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