Which component is the enzyme in the ABC complex?

• A. Horseradish peroxidase
• B. Alkaline phosphatase
• C. Luciferase
• D. Beta-galactosidase

Answer: (A)

In the ABC (Avidin-Biotin Complex) method, the enzyme carried by the complex is horseradish peroxidase. The sequence is: a biotinylated secondary antibody binds the primary antibody that has recognized the target antigen; the avidin in the complex binds to those biotin sites on the antibody, bringing along horseradish peroxidase to the antigen site. HRP then uses a chromogenic substrate like DAB to produce a visible brown precipitate, revealing where the antigen is located. This system benefits from signal amplification because avidin (with multiple biotin-binding sites) can carry multiple enzyme molecules, enhancing sensitivity. Alkaline phosphatase, luciferase, and beta-galactosidase are enzymes used in other detection setups, but the classic ABC-based chromogenic IHC uses horseradish peroxidase.