The C3a receptor also known as complement component 3a receptor 1 (C3AR1) is a G protein-coupled receptor protein involved in the complement system.Although its official name implies it only binds C3a, it also binds C4a.Crass et al. (1996) isolated a cDNA encoding C3AR1 from an expression library prepared from U-937 cells that had been differentiated with dibutyryl cAMP to a macrophage-like phenotype. The predicted C3AR1 protein has 7 hydrophobic membrane-spanning helices and shares high homology with C5AR and the formyl-met-leu-phe receptor (FPR1). C3AR1 contains an exceptionally large second extracellular loop composed of approximately 175 residues. Northern blot analysis detected major 2.3- and minor 3.9-kb C3AR1 transcripts.