The HK5.3 antibody reacts with mouse CD275, an approximately 40 kDa protein that is a member of the Ig superfamily. CD275 is also known as B7-H2, ICOS ligand, and B7-RP1. CD275 is a ligand for ICOS (CD278) that is expressed on activated T cells. CD275 is expressed by antigen presenting cells including B cells, macrophages, and dendritic cells. The interaction of CD275 with its receptor ICOS plays an important role in T cell costimulation.