The
cluster of differentiation (
cluster of designation or
classification determinant) (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 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.
CD for humans is numbered up to 364 .