Cluster of differentiation

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The CD cell surface molecules are recognized by specific sets of antibodies.

Cells are the fundamental unit of life and an extraordinary array of specialized cells have evolved to accomplish the many different tasks required of organisms in their environments. In order for specialized cells to interact with each other and their environments, they must bring things in, release other things, sense pressure, and, in some cases, recognize other cells and present markers that allow other cells to recognize them. All of these functions require cells to have lots of unique molecules on their surfaces. Imagine a pin cushion with thousands and thousands of pins: some big, some small; some sticky; some have flat heads, some have round heads, some are pushed all the way in, some are sticking way out; some of the pins are needles, little pipes, channels that let things into and out of the cell. Cluster of differentiation (CD) molecules are a defined subset of cell surface molecules. They are found on the surfaces of various cells and can be recognized by specific sets of antibodies. These antibodies can be used to identify the cell type, stage of differentiation and activity of a cell. Unlike many of the descriptive words used for biological molecules, like transporters, receptors, the CD nomenclature was developed empirically by immunology researchers. To understand the CD nomenclature, one should first learn about a bit about antibodies.

1 Nomenclature
2 Uses
3 See also
4 External links

[edit] 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. This system was intended for the classification of the many monoclonal antibodies (mAbs), generated by different laboratories around the world, against various cell surface molecules (epitopes) on leukocytes (white blood cells). Since then, the use has expanded to many other cell types, and more than 250 CD clusters and subclusters have been identified. The HLDA workshops assign each CD based on the same reactivity to one human antigen by at least two mAbs; the provisional indicator "w" (as in "CDw186") is sometimes given to a cluster not well characterized or represented by only one mAb.

[edit] Uses

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; for example, a "CD34+, CD31–" cell is one that expresses CD34, but not CD31. This would typically correspond to a stem cell, rather than a fully differentiated endothelial cell.

The most commonly referred to CD molecules are CD4 and CD8, which are markers for two different subtypes of T-lymphocytes, for the most part (dendritic cells also express CD8). CD4 is specifically recognized and bound by HIV, leading to viral infection and 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.

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. In the example of CD4 & CD8, these molecules are critical in the antigen recognition pathway.