Which assay uses a labeled antibody to detect an antigen within a tissue sample?

• A. Indirect immunofluorescence
• B. Direct immunofluorescence
• C. Western blot
• D. ELISA

Answer: (B)

Direct immunofluorescence uses a fluorophore-labeled antibody that binds directly to the antigen within a tissue section, allowing visualization of exactly where the antigen is located in the tissue under a fluorescence microscope. This single-step labeling provides in situ localization with a straightforward workflow and typically less background from secondary antibodies. In contrast, indirect immunofluorescence uses a primary antibody to bind the antigen and a separate labeled secondary antibody to generate the signal, which increases sensitivity but adds extra steps. Western blot detects proteins on a membrane after separation, losing tissue context, and ELISA measures antigens in solution or on a plate rather than within tissue sections.