What are Cluster of Differentiation?Cluster of differentiation (CD) are a group of proteins located on cell surface. They are found in all cell types but are predominantly expressed on the leukocytes. CD proteins are often used as markers to identify different cell phenotypes. Most of these molecules also play important cell functions ranging from cell signaling to cell adhesion and antigen recognition. These proteins may be specific to one type of cell or cell lineage although different cells may contain the same CD protein. Cells may contain different CD markers based on their stage of maturation. Many specific diagnostic agents have been developed to identify different CD proteins which facilitate the isolation and identification of specific cell types. CD proteins have a wide range of applications. For example, CD markers are used in the classification of white blood cells and have diagnostic purposes. More than 400 CD proteins have been identified so far.
Brief history of CD proteinsCD proteins were initially developed as potential leucocyte markers. Nowadays they are used to identify and investigate many types of cells. Several antibodies have been developed to detect specific CD markers. Nomenclature is important to distinguish between antibodies and CD markers and to identify which antibodies are against the same molecule. CD nomenclature was established in 1982 during the first international Workshops known as the Human Leucocyte Differentiation Antigens (HLDA) held in Paris. The first HDLA Workshop evaluated many of CD antibodies by conducting multi-laboratory blind analysis of antibodies. This approach provided an independent method of validation of antibody usability and specificity and has increased the confident use of these reagents in research. The current CD nomenclature derives from the first HDLA. A series of HDLA Workshops have taken place since to organize the studies of these CD surface molecules. Since the first HDLA, CD unique clusters have been identified in different cell types and not just leucocytes.
What are CD antigens?CD proteins are often used as cell markers in H3immunophenotyping H3. Immunophenotyping is a technique that uses antibodies to recognize specific CD markers. CD proteins in this case act as epitopes and are often referred as antigen markers. These markers are often used to associate cells with certain immune functions. The use of one CD molecule to define populations of cells is uncommon whereas a combination of different CD proteins is often used to identify and characterize several types of cells or cell lineage.
CD proteins are especially useful for the identification of leucocytes using flow cytometry. The latter is a technology that rapidly analyzes single cells or particles as they flow past single or multiple lasers based on the CD molecules on their surface. One of the main advantages of flow cytometry is that it can detect simultaneously several markers on a single cell at the very same time. As a result, flow cytometry has numerous applications in medicine and research.
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