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.