C3a is an anaphylatoxin generated throμgh activation of the complement C3. C3a is a potent chemoattractant for phagocytes, and it stimulates chemotaxis and other leukocyte functions. C3a binds to eosinophils, basophils, neutrophils, and differentiated U937 cells, that all express receptors for C3a on their cell surface. The C3a receptor has been cloned and shown to be a G protein-coupled receptor with seven putative transmembrane domains. This receptor is characterized for its large extracellular loop between the fourth and fifth transmembrane domains. The loop can be readily detected using the rabbit anti-C3a receptor antibody.