In direct immunohistochemistry staining, what is used to identify antigens in the tissue?

• A. Labeled antibody of known specificity
• B. Patient’s serum
• C. ABC complex
• D. Biotin-streptavidin

Answer: (A)

Direct immunohistochemistry uses a labeled antibody that has proven specificity for the antigen and carries the detectable tag itself, such as an enzyme or fluorophore. This primary antibody binds directly to the target in the tissue, allowing visualization without a secondary reagent. The other options fit indirect methods: patient serum isn’t a controlled, labeled probe for a specific tissue antigen, and the ABC complex or biotin-streptavidin systems rely on a secondary antibody and amplification steps rather than a single labeled primary antibody.