Cluster of differentiation (CD) are proteins that are usually surfaces of white blood cells and are hence used for research investigations of certain aspects of the immune system. Their primary function is to act as receptors or ligands in cell signaling and as cell adhesion molecules and are therefore situated on an exposed part of the cell. There are more than 300 known CD in humans for example. Because of this they are often used to distinguish cell types and for selection purposes, for example by usage of flow cytometry. In some cases individual CD can be used to distinguish cells from each other. In other cases it is necessary to analyze various CD on a group of cells to establish a specific pattern of CD. The nomenclature of CD commonly follows the pattern “CD31”, for cluster of differentiation followed by an identification number. An addition of a plus or minus (+ / -) sign indicates if a certain CD is present in a population of cells or not. Examples for common CD are CD4 and CD8 which are used as markers of helper and cytotoxic T-cells, respectively. These CD have become known more widely due to the fact that they are involved in the infection progression of HIV.