TRIM21/Ro52, an intracellular autoantigen implicated in Sjögren's disease, is released from cells undergoing lytic death (pyroptosis/necroptosis) but not apoptosis, where it binds circulating antibody Fc and F(ab')2 domains to form large proinflammatory immune complexes. Flow Induced Dispersion Analysis (FIDA), on a Fida 1 instrument, was used to measure the hydrodynamic radius of TRIM21–antibody complexes, revealing sizes of 17–53 nm with a control antibody and up to 17–212 nm with Sjögren's disease patient plasma. These immune complexes were taken up by macrophages and drove pro-inflammatory and antigen-presentation responses in high-interferon environments. It is an example of using FIDA to characterize the size and oligomeric state of antibody–antigen immune complexes.