Fibrinogen-containing circulating immune complexes (ICs) in rheumatoid arthritis (RA). Circulating (a) IgG and (b) IgM ICs were detected in plasma derived from healthy individuals and anti-cyclic-citrullinated peptides (CCP) – and anti-CCP+ RA patients. ELISA plates were coated with C1q, incubated with 1:50 dilutions of plasma samples and horseradish peroxidase (HRP)-conjugated (a) anti-IgG or (b) anti-IgM secondary antibodies were used to detect the immunoglobulin isotypes contained in ICs. (c, d) Circulating fibrinogen-containing ICs were detected using HRP-conjugated fibrinogen-specific antisera as the secondary reagent. Statistical comparisons are based on an unpaired t-test with Welch correction. (e) Fibrinogen-containing ICs were detected with different dilutions of RA patient samples and healthy controls. Error bars represent the standard deviation of results from triplicate wells. (f) Fibrinogen-containing ICs were detected in fresh plasma and freeze-thawed plasma samples with no significant differences in values (data for anti-CCP status of RA5 is not available).