B7-H3 is an immunoglobulin superfamily protein that is expressed by osteoblasts, fibroblasts, ciliated respiratory epithelial cells, anterior pituitary progenitor cells, first trimester extravillous trophoblasts, and multiple tumor cells. It is induced on activated dendritic cells (DC), skeletal muscle myocytes, and monocytes. B7-H3 can both stimulate and inhibit immune cell activation. As a co-stimulator, B7-H3 augments macrophage activation and promotes proliferation and cytokine secretion by CD4+ and CD8+ T cells. As a co-inhibitor, tumor cell B7-H3 blocks the cytolytic activity of NK cells, limits cytokine production by activated T cells, and imparts an immunosuppressive phenotype to dendritic cells. B7-H3 can interact with TREML2/TLT-2 to enhance T cell activation including the CD8+ T cell-mediated lysis of tumor cells.