May participate in the regulation of T-cell-mediated immune response. May play a protective role in tumor cells by inhibiting natural-killer mediated cell lysis as well as a role of marker for detection of neuroblastoma cells. May be involved in the development of acute and chronic transplant rejection and in the regulation of lymphocytic activity at mucosal surfaces. Could also play a key role in providing the placenta and fetus with a suitable immunological environment throughout pregnancy. Both isoform 1 and isoform 2 appear to be redundant in their ability to modulate CD4 T-cell responses. Isoform 2 is shown to enhance the induction of cytotoxic T-cells and selectively stimulates interferon gamma production in the presence of T-cell receptor signaling. B7-H3 locus underwent genomic duplication leading to tandemly repeated immunoglobulin-like V and C domains (VC domains). The dominantly expressed human B7-H3 isoform contains tandemly duplicated VC domains. In contrast, mouse B7-H3 transcript contains only one single VC domain form due to an exon structure corresponding to V domain- (pseudoexon C)- (pseudoexon V)-C domain. This duplication appearing in primates is suggested to be very recent supporting a model of multiple independent emergence of tandem VC repeats within human and monkey species.