C5AR1 and Arthritis: C5a levels in synovial fluid are elevated in patients with RA [16], a genome-wide association study (GWAS) indicated that the TRAF1-C5 region is related to RA in humans [17, 18], numerous animal models have indicated that C5 is a gene responsible for causing arthritis [19, 20], and C5 and C5a receptor knockout mice are resistant to arthritis [21, 22].