The monoclonal antibody 17 reacts with the human receptor for C3a. The monoclonal antibody 17 is in the literature also referred to as clone hC3aRZ8.In the course of complement activation the C3a receptor functions as the cell surface receptor for the anaphylatoxin C3a, the C-terminal 77 amino acid cleavage product of the alpha chain of C3, but not for C3a-desArg. All cellular responses to C3a are specifically mediated by interactions with the membrane bound C3a receptor, a seven transmembrane GTP-binding-protein-coupled receptor that belongs to the rhodopsin supergene family. The 54 kDa C3a receptor is widely distributed in, myeloid cells, peripheral tissues and the central nervous system. Expression of C3aR has been demonstrated on neutrophils, monocytes, eosinophils, astrocytes, neurons and glial cells and is increased in inflammatory conditions. No C3aR expression was detectable on lymphocytes and on tonsillar B cells even after stimulation.