The immune system is composed of a variety of cell types each with unique roles and functions. These different cell types can be identified by a unique signature of cell-surface proteins called CD (cluster of differentiation) markers. Well known examples of CD markers are CD4 and CD8, which are used to identify T lymphocytes involved in regulating the adaptive immune response. CD markers for flow cytometry are used in many applications, including immunophenotyping (see below).
The CD nomenclature has been developed and maintained by the HLDA (Human Leukocyte Differentiation Antigens) workshop. This initiative, starting in 1982, originally aimed to classify the monoclonal antibodies (mAbs) raised against cell surface molecules of leukocytes being generated by laboratories worldwide. The proposed marker is assigned a CD number once two specific mAbs have been shown to bind to it. Markers that are not sufficiently characterized are labeled with a provisional “w” (as in “CDw145”).
There are now over 370 CD unique clusters and subclusters described in humans, including cell types other than leukocytes. CD antibodies are used widely for studying cell structure, function and distribution in research, differential diagnosis, and monitoring and treatment of disease (Table 1).