B7-H2, a member of the B7 family and the immunoglobulin superfamily, is a 40kD protein also known as B7RP-1, B7h, B7-H2, GL50 and ICOS Ligand. Human B7-H2 is expressed by activated monocytes/macrophages and dendritic cells. B7-H2 binds to a CD28-like receptor, inducible costimulator molecule (ICOS, AILIM, CRP-1), which is expressed by activated T cells. The interaction of ICOS with B7-H2 plays an important role in the T cell costimulation pathway.