The "complement system" is a critical part of body's defense mechanism against pathological infections. One of the most potent inflammatory peptides of the complement system, the C₅a, exerts its effect by binding to, and activating a G Protein-Coupled Receptor, the C₅aR. The interaction of C₅a and C₅aR and downstream signaling is critical for rapid response to pathogenic infections. Here, we propose to crystallize and determine the crystal structure of C₅a-C₅aR complex to reveal the structural basis C₅a-C₅aR interaction. This novel information should improve our understanding of C₅a-C₅aR signaling system and provide a structural framework to facilitate novel drug discovery and design for sepsis and inflammation.