CD antigens mean the cell surface antigens of leukocytes. When lymphocytes mature, they express diverse protein receptors on the cell surface, and the protein receptors are useful in determining the type and maturation stage of the cells being examined. Some CD proteins do not play a role in cell signaling, but have other functions, such as cell adhesion.
CD antigens actually are subpopulations and functional types of leukocytes. They take part in immune reaction as receptors for cell communication. Though only a fraction of CD molecules have been known well, most of them have an important function. In the example of CD4 and CD8, these molecules are critical in antigen recognition. Others (e.g., CD135) act as cell surface receptors for growth factors.