Citrullinated fibrinogen-containing immune complexes (ICs) are separated from rheumatoid arthritis (RA) plasma. (a) Liquid chromatographic separation of RA plasma. Fast protein liquid chromatography (FPLC) was used to fractionate plasma derived from RA and control patients. Forty-five fractions were collected from each plasma sample, and individual fractions were analysed for total protein, fibrinogen, IgG, IgG ICs and fibrinogen ICs (FIC), and relative levels of each of these components are plotted. Plasma samples from two RA patients (RA1 and RA2), a psoriatic arthritis (PsA) patients and a healthy control were characterised. The right panel presents individual traces from patient RA1, with the dashed lines indicating the fractions containing the peak levels of ICs, free fibrinogen and free Ig. (b) Citrullinated fibrinogen was identified by anti-modified citrulline blot. (c) In-gel trypsin digestion of the bands followed by mass spectrometry revealed two citrullinated peptides derived from beta chain of human fibrinogen.