What is the primary function of blocking proteins in immunohistochemistry?

• A. Block non-specific protein adsorption by occupying binding sites on the tissue.
• B. Improve antigen retrieval efficiency.
• C. Enhance chromogen development by acting as co-factors.
• D. Preserve tissue morphology during staining.

Answer: (A)

Blocking proteins prevent non-specific binding by occupying binding sites on the tissue, so antibodies bind mainly to the intended antigen. By coating reactive sites with proteins such as normal serum from the antibody host, BSA, casein, or gelatin, background staining is reduced and signal specificity improves. This step comes before antibody incubation to keep the staining clean. It isn’t about enabling antigen retrieval, aiding the chromogenic reaction, or preserving tissue morphology—those are handled by separate processes (epitope unmasking, detection chemistry, and fixation/processing, respectively).