The most commonly known CD antigens are CD4 and CD8 which are markers for T-helper and T-suppressor cells, respectively. CD4 binds to relatively invariant sites on class II major histocompatibility complex molecules outside the peptide-binding groove, which interacts with the T-cell receptor. CD4 is also the central docking receptor for human immunodeficiency virus. CD8 binds to relatively invariant sites on class I major histocompatibility complex molecules outside the peptide-binding groove. CD8 is also expressed on a subset of dendritic cells. Other more important CD antigens include the leukocytes integrins (CD11/CD18) and the hematopoietic stem cell marker CD34.

CD Antigens are not merely markers on the cell surface, but also 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 cluster of differentiation antigens do not play a role in cell signaling, but have other functions, such as cell adhesion.

CD markers not only play a key role in the diagnosis of cancer, they also can help identify which types of treatment may be most successful and measure how effective the treatment is by monitoring changes in the relevant CD markers. Moreover, researchers are today able to create a type of defensive protein, known as a monoclonal antibody (mAb), which is matched to a specific CD antigen. These cloned antibodies mimic those produced by the body and can be used to fight cancer in a form of treatment known as targeted immunotherapy.