a type I transmembrane protein present on myeloid cells, platelets, endothelial cells, natural killer cells, monocytes, and subsets of CD4-positive T cells that acts as a cosignal transducer for macrophages, inducing respiratory burst; plays a key role in the transmigration of leukocytes through intercellular junctions of vascular endothelium and mediates calcium-dependent heterophilic aggregation; expressed in neoplastic endothelial cells.