What is the primary purpose of blocking steps in IHC?

• A. To prevent non-specific binding
• B. To enhance antigen retrieval
• C. To label antibodies with enzymes
• D. To dissolve tissue sections

Answer: (A)

Blocking steps are used to prevent non-specific binding of antibodies to tissue components. Tissues contain many sites that can attract antibodies besides the target antigen, leading to background staining that hides the specific signal. Applying blocking agents such as serum from the secondary antibody’s host, BSA, casein, or commercial blockers saturates those non-specific sites so the antibodies bind mainly to the intended antigen. This greatly improves specificity and the clarity of the staining. Endogenous enzyme activity can also be blocked to avoid unwanted color development, but that’s a separate consideration from blocking non-specific binding. Antigen retrieval, on the other hand, serves to unmask epitopes, not to reduce non-specific binding, and blocking is not about labeling antibodies or dissolving tissue.