The link between tissue-bound primary Ab and the avidin-biotin-peroxidase complex is formed by which interaction?

• A. Biotin-Streptavidin
• B. Primary Ab-Ag direct
• C. DAB reaction
• D. Fluorescent tag

Answer: (A)

The linking event is the binding between biotin and streptavidin. In the ABC (avidin–biotin–peroxidase) system, a biotinylated secondary antibody sits on the tissue-bound primary antibody. Then streptavidin conjugated to peroxidase binds tightly to that biotin, bringing the enzyme right to the antigen site. The extremely high affinity of the biotin–streptavidin interaction provides a robust bridge that enables the chromogenic reaction to reveal the target. Other options describe the substrate reaction or a different detection modality (direct labeling or fluorescence) and do not serve as the bridging link between the tissue-bound antibody and the detection complex.