The cluster of differentiation (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 signalling, but have other functions, such as cell adhesion.
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