Rheumatoid arthritis (RA) is a systemic inflammatory autoimmune disease, characterized by chronic, erosive polyarthritis and by the presence of various autoantibodies in serum and synovial fluid. Since rheumatoid factor (RF) was firstly described, a number of other autoantibodies have been discovered in RA patients. The autoantigens recognized by these autoantibodies include cartilage components, chaperones, enzymes, nuclear proteins and citrullinated proteins.
In 1993, Serre et al (1993; 92:1387–1393) identified filaggrin as the target antigen of RA-specific anti-keratin antibodies. Subsequently, it has been demonstrated that anti-keratin antibodies and other RA-specific autoantibodies known as anti-perinuclear factors and anti-Sa antibodies all recognize citrulline-containing peptides/proteins as common antigenic entity, and they are collectively termed as anti-citrullinated protein antibodies. Currently, only RF and anti-citrullinated protein antibodies are utilized in clinical practice because of their diagnostic and prognostic values as well as they are highly specific for RA.