Complement is necessary in arthritis development in the collagen-induced arthritis (CIA) model [15], and an active role of C5a has also been implicated in other arthritis models: C5aR deficiency protects against arthritis in collagen antibody-induced (CAIA), the chronic autoimmune SKG, and the K/BxN serum transfer models [16–19], while anti-C5a or–C5aR blockade is efficacious in collagen-induced (CIA) and rat antigen-induced arthritis [20, 21]. The gene discussed is C5; the disease is arthritic joint disease.