Human B7-H2 is a member of the growing B7 family of immune costimulatory proteins. Other family members include B7-1, B7-2, B7-H1 (PD-L1), PD-L2 and B7-H3. B7 proteins are members of the immunoglobulin (Ig) superfamily, the extracellular domains contain 2 Ig-like domains and all members have short cytoplasmic domains. B7-H2 has been identified as the ligand for ICOS, a member of the CD28 family of costimulatory receptors. Human B7-H2 is expressed constitutively on resting B cells, dendritic cells and at low levels on monocytes. The B7-H2/ICOS interaction appears to play roles in T cell dependent B cell activation and Th differentiation.