In the PAP method, what does the PAP complex consist of?

• A. Antibodies against the secondary antibody conjugated with peroxidase plus anti-peroxidase antibody
• B. Primary antibody alone
• C. The chromogen substrate
• D. A fluorescent label

Answer: (A)

In the PAP method, the signal is built by forming a PAP complex at the site where the primary antibody has bound the antigen. This complex consists of antibodies against the secondary antibody that are themselves conjugated to peroxidase, together with anti-peroxidase antibodies. Placing these components together brings multiple peroxidase enzymes to the antigen locus, so when a chromogenic substrate is added, a strong localized color develops. The chromogen substrate and a fluorescent label are not what make up the PAP complex themselves; the chromogen is just the dye produced by the enzyme, and fluorescent labels are used in different detection systems.