Cell surface antigens of leukocytes are called CD antigens, and important for immune reactions of organisms. As lymphocytes mature, they express different protein receptors on the cell surface, which can aid in determining the type and maturation stage of the cells being examined. These proteins or antigen markers are called Clusters of Differentiation. The term CD means a cluster of differentiation OR a cluster of determinants which indicates the lineage or maturational stage of lymphocytes.During the course of development from precursor cells into functionally mature forms, lymphocytes display a complex pattern of surface antigens, some of which are acquired at certain stages while others are lost.