While 90% of neutrophils located in the lumen of synovial blood vessels were C5aR-positive, as determined by double staining for MPO and C5aR, only 5% of the neutrophils infiltrating the synovium from either RA or OA patients undergoing joint replacement were C5aR+ (Fig 2E). This evidence concerns the gene C5AR1 and rheumatoid arthritis.