The RPA-2.10 antibody reacts with CD2, an ~50 kDa type I transmembrane glycoprotein and a member of the immunoglobulin (Ig) superfamily; two Ig-like domains are located in its extracellular portion. CD2 is broadly expressed on peripheral T and NK cells, dendritic cells, erythrocytes, most thymic T cells, subsets of thymic B cells, and on the endothelium. CD2 expression appears early during T cell differentiation. Aberrant expression has been observed in some lymphomas and myeloid leukemias. CD2 is critically important for T cell activation and signaling, as well as lymphocyte adhesion. The primary ligand for CD2 is CD58 (LFA-3) located on antigen-presenting cells, with additional ligands comprising CD15 (SSEA-1), CD48 and CD59. Notably, the RPA-2.10 antibody blocks the mixed lymphocyte reaction.