Autoimmunity in rheumatoid arthritis (RA) is characterised by an antibody response to citrullinated proteins. Periodontitis (PD) is largely caused by infection, in whichPorphyromonas gingivalis is a major pathogen. The two diseases combine specific HLA-DRB1alleles and smoking as risk factors, and have a similar pathophysiology characterised by destructive inflammation. A possible causative link be ...