CD Antigen / CD Antigens

CD Antigen Background

The CD antigens are membrane proteins mainly expressed on leukocytes. A small number are also expressed on endothelial cells, erythrocytes, and stem cells. The CD antigens are commonly used as cell markers, allowing the type and maturation stage of the cells to be defined based on what molecules are present on their surface. The majority of CD antigens perform critical functions in immune responses of organisms. Because of their common expression at the leukocyte surface and their critical roles in the human immune response, detection the expression of CD antigens is supposed to be developed as diagnosis methods in many human diseases.

The term CD means Cluster of Differentiation, which was proposed for the classification of the many monoclonal antibodies (mAbs) generated by different laboratories around the world against epitopes on the surface molecules of leukocytes. Since then, the use of CD has expanded to many other cell types, and for humans CD is numbered up to 350 most recently (as of 2009). A small letter w before the number designation stands for "workshop", which indicates that the CD designation is tentative.

The CD antigens are commonly used as cell markers, allowing cells to be defined based on what molecules are present on their surface. For example, two commonly-used CD antigens are CD4 and CD8, which are, in general, used as markers for two different subtypes of T-lymphocytes, helper and cytotoxic T cells, respectively. CD4 is specifically recognized and bound by HIV, leading to viral infection and destruction of CD4+ T cells. The relative abundance of CD4+ and CD8+ T cells is often used to monitor the progression of an HIV infection. Other commonly used CD markers include CD3, CD31, CD34, CD45, CD16, CD19, CD20, CD56, etc.

CD antigens are not merely markers on the cell surface. CD antigens can act in numerous ways and are important for immune reactions of organisms. They often act as receptors or ligands important to the cell, initiating a signal cascade and altering the behavior of the cell. Some CD antigens do not play a role in cell signaling, but have other functions, such as cell adhesion.

CD antigens function as Fc receptors such as CD16, CD32, CD89, CD64, etc. There are also CD antigens that act as receptors for cytokines, such as CD30, CD40, CD27, and CD95. Some CD antigens act as co-stimulatory molecules such as CD28, CD80, CD86, and CD150. CD antigens play critical role in cell adhesion such as CD50, CD54, CD102, CD166, CD112, CD146, etc.

CD antigens are usually characterized as molecules expressed on the cell surface, but some CD antigens are actually transmembrane proteins. These CD antigens include members of the receptor tyrosine kinases, CD331, CD220, and CD309.

The human CD antigens constitute a promising assay content for antibody microarray applications, because of their common expression at the leukocyte cell surface and the fact that the majority perform critical functions in the human immune response. The diagnostic potential of a microarray, containing 82 CD monoclonal antibodies (DotScan microarrays) has been demonstrated for a variety of infectious and neoplastic disease states, including HIV, many acute and chronic leukemias, and colorectal cancer.