Toluidine blue is used to demonstrate which cells?

• A. Plasma cells
• B. Mast cells
• C. Fibroblasts
• D. Macrophages

Answer: (B)

Toluidine blue shows metachromasia in cells that contain highly acidic granules. Mast cells have granules rich in sulfated glycosaminoglycans (like heparin). When stained with toluidine blue, these granules shift color from blue to a purple or reddish-purple, making mast cells stand out against the surrounding tissue. Other cells (such as plasma cells, fibroblasts, or macrophages) don’t have these metachromatic granules, so they don’t display the distinctive color change.