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).

Examples of CD Markers

Marker Cell Type
CD2, CD3, CD4, CD5, CD7, CD8 T cells
CD10 Early pre-B cells (immature B cells)
CD11c, CD25, CD103, CD123 Hairy cell leukemia cells
CD13, CD33, CD117 Myeloid cells
CD14, CD64 Monocytic cells (positive in AML-M4 and AML-M5)
CD15 Reed-Sternberg cells, neutrophils
CD16, CD56 Natural killer cells
CD19, CD20, CD21, CD22 B cells
CD23 and CD5 Chronic lymphocytic leukemia/small lymphocytic lymphoma
CD31 Endothelial cells (positive in angiosarcoma), megakaryocytes and platelets
CD33 Myeloid cells and precursors
CD34 Stem cells (also positive in angiosarcoma)
CD41, CD61 Megakaryocytes and platelets (positive in AML-M7)
CD45 All leukocytes (except Reed-Sternberg cells)
CD45 RO Memory T cells
CD45 RA Naive T cells
CD56 Natural Killer (NK) cells
CD79b B cells, plasma cells, and lymphoproliferative disorders
CD200 A useful marker to differentiate between CLL (positive) and MCL (negative)

Table 1.

See a full list of CD markers for flow cytometry