Cluster of Differentiation (CD) antigens are defined when a surface molecule found on some members of a standard panel of human cells reacts with at least one novel antibody, and there is good accompanying molecular data. Leucocyte cell surface molecules mediate many of the interactions between the immune system and antigens, between components of the immune system, and between the immune system and other physiological systems, such as the nervous system.
The Human Leukocyte Differentiation Antigens (HLDA) Workshops were initiated1 to bring order to the chaos that existed in 1984, as immunologists generated large numbers of monoclonal antibodies reactive with leukocyte cell-surface molecules, each with different associated nomenclatures. This analytical method identified clusters of antibodies with very similar patterns of binding to leukocytes at various stages of differentiation: hence the "cluster of differentiation" (CD) nomenclature. This provided a common nomenclature that allowed the scientific community to communicate results in a universal language. Studies of immunity and diseases of the immune system have been greatly facilitated over the last 30 years by the HLDA Workshops which have used antibodies to characterise many of the molecules involved in immunological processes, and have provided a nomenclature system, the CD system, which is used universally.
CD molecules are defined or classified by the reference monoclonal antibodies to which they bind. There are many clusters of differentiation; many cells express more than one CD marker. A phenotypic characterization of a cell of hematopoietic origin may be made by examining the pattern of CD markers expressed at any given time. The physiologic function has been identified for many, but not all, known CDs.
The cluster of differentiation (often abbreviated as CD) is a protocol used for the identification and investigation of cell surface molecules providing targets for immunophenotyping of cells. In terms of physiology, CD antigens 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. Some CD antigens do not play a role in cell signaling, but have other functions, such as cell adhesion, cell activation, and cell inhibition.
The CD antigens are commonly used as cell markers in immunophenotyping, allowing cells to be defined based on what molecules are present on their surface. These markers are often used to associate cells with certain immune functions. While using one CD molecule to define populations is uncommon (though a few examples exist), combining markers has allowed for cell types with very specific definitions within the immune system.
CD antigens are utilized in cell sorting using various methods including flow cytometry.
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