ICOS ligand is a member of the B7 family and the immunoglobulin superfamily. Human ICOS ligand is expressed by activated monocytes/macrophages and dendritic cells. It binds to a CD28 like receptor, inducible costimulator molecule (ICOS, AILIM, CRP-1), which is expressed by activated T cells. This interaction plays an important role in the T cell costimulation pathway.