In fluorescence immunohistochemistry, what causes positive specimens to glow?

• A. Visible light
• B. Infrared
• C. No light
• D. UV light

Answer: (D)

Fluorescence comes from fluorophores absorbing energy from an excitation source and then re-emitting light at a longer wavelength. In immunohistochemistry, the antibody is tagged with a fluorophore that binds specifically to the target. When you illuminate the slide with UV light, the fluorophore absorbs those photons and releases visible photons as it returns to the ground state. That emitted light is what makes positive specimens glow. UV is a common excitation for many fluorescent dyes, whereas simply using visible light or no light wouldn’t selectively trigger emission, and infrared excitation would rely on different dyes.