In immunohistochemistry (IHC), quenching primarily targets endogenous enzymes and molecules in tissues that can interfere with detection reagents. For example, many tissues contain endogenous peroxidase, which can react with horseradish peroxidase (HRP)-based detection systems, causing false-positive staining. To prevent this, a Peroxidase Block using hydrogen peroxide (commonly 3%) is applied after antigen retrieval. This step inactivates the endogenous peroxidase enzyme, ensuring that only the HRP linked to the antibody produces signal. Similarly, alkaline phosphatase activity can be quenched using reagentsinfectious reagents like levamisole. Additionally, tissues may contain endogenous biotin, which can bind avidin or streptavidin used in some detection systems, causing background staining. To address this, an Avidin/Biotin Blocking Kit is used to saturate endogenous biotin and avidin binding sites before antibody incubation.
Blocking to Prevent Nonspecific Binding
Following quenching, blocking steps are performed to prevent nonspecific antibody binding to tissue proteins or Fc receptors, which can cause diffuse background staining. Protein Block Solutions containing bovine serum albumin (BSA), casein, or commercial proprietary buffers saturate hydrophobic and ionic binding sites on tissue proteins. Blocking sera, usually normal serum from the species in which the secondary antibody is raised (e.g., goat serum for goat secondary antibodies), block Fc receptors and other nonspecific sites. This step is critical to reduce background and improve the signal-to-noise ratio.
The Liaison Between Quenching and Blocking
Together, these steps form a coordinated strategy: quenching neutralizes endogenous enzymatic activities and biotin, while blocking prevents nonspecific antibody interactions. Typically, quenching is done immediately after antigen retrieval, followed by blocking before primary antibody application. This liaison ensures that IHC staining is specific, clear, and reproducible, minimizing false positives and background noise that could obscure true antigen detection.
