The higher tendency of the levels of C5a in serum from the group of RA patients with negative Treponema denticola, Tannerella forsythia, and Aggregatibcter actinomycetemcomitans antibody was observed compared with serum from the group of positive Treponema denticola, Tannerella forsythia, and Aggregatibcter actinomycetemcomitans antibody patients. Here, C5AR1 is linked to rheumatoid arthritis.