AIM To assess potential contributions of biliary IgA for crystal agglomeration into gallstones, we visualized cholesterol crystal binding of biliary IgA.METHODS Crystal-binding biliary proteins were extracted from human gallbladder bile using lectin affinity chromatography. Biliary IgA was isolated from the bound protein fraction by immunoaffinity chromatography. Pure cholesterol monohydrate crystals were incubated with biliary IgA and fluoresceine isothiocyanate (FITC)-conjugated anti-lgA at 37C. Samples were examined under polarizing and fluorescence light microscopy with digital image processing.RESULTS Binding of biliary IgA to cholesterol monohydrate crystals could be visualized with FITC-conjugated anti-lgA antibodies. Peak fluorescence occurred at crystal edges and dislocations. Controls without biliary IgA or with biliary IgG showed no significant fluorescence.CONCLUSION Fluorescence light microscopy provided evidence for cholesterol crystal binding of biliary IgA. Cholesterol crystalbinding proteins like IgA might be important mediators of crystal agglomeration and growth of cholesterol gallstones by modifying the evolving crystal structures in vivo.