CD97 is a seven-span transmembrane protein belonging to the secretin receptor superfamily, the G-protein-coupled receptor superfamily, and the EGF-TM7 family with molecular weights of 74 kD, 80 kD, and 86 kD. It is expressed on monocytes/macrophages, granulocytes, dendritic cells, and smooth muscle cells.The expression of CD97 on resting T- and B-lymphocytes is at low level, but is rapidly upregulated upon activation. CD97 binds to CD55 (decay-accelerating factor), chondroitin sulfate, integrin a5b1, and a3b1. CD97 has been shown to mediate cell adhesion and co-stimulation of T cell proliferation.