The RPA-T8 monoclonal antibody specifically reacts with the human CD8a molecule, a 32 kDa cell surface receptor expressed either as a heterodimer (CD8 α/β) or as a homodimer (CD8 α/α) on the majority of thymocytes, a subpopulation of mature T cells, and natural killer cells. CD8 interacts with the major histocompatibility complex class I (MHC class I) molecules on antigen-presenting cells or epithelial cells. The RPA-T8 antibody reacts with 13-48% of peripheral lymphocytes, 80% of thymocytes, and a subset of natural killer cells.

RPA-T8, OKT8, and HIT8a antibodies do not compete with each other for binding to peripheral leukocytes, meaning that that they do not recognize the same epitope or block each other by steric hindrance.