The effect of a protein meal on three fluid-phase assays for circulating immune complexes

Serial blood samples were taken from eight patients with systemic lupus erythematosus and five normal controls before and after a standard meal containing 20 g of protein. The samples were assayed for circulating immune complexes by the fluid-phase C1q, monoclonal rheumatoid factor and conglutinin-binding assays. No significant differences were seen in the post- as compared to the pre-prandial levels of circulating immune complexes. In neither controls nor patients did circulating complexes appear for the first time within 90 min after starting the meal. We conclude that if systemic antigen--antibody reactions occur soon after a protein meal they are not detected by the polyethylene glycol-dependent tests in current use and do not appear to influence the immediate clearance of endogenous immune complexes.