Blocking peptides added in excess during IHC result in

• A. Antibody is blocked and cannot bind to the target
• B. Increased antibody affinity for the target
• C. Enhanced antigen retrieval
• D. Increased signal intensity

Answer: (A)

Blocking peptides compete with the tissue antigen for the antibody’s binding site. When added in excess, the soluble peptide saturates the antibody’s paratope, so the antibody binds the peptide rather than the target in the tissue. That means the antibody is effectively blocked from binding its antigen, and staining is abolished. This does not increase intrinsic antibody affinity, nor does it enhance antigen retrieval, and it would reduce signal intensity rather than increase it. Using blocking peptide this way can help confirm antibody specificity: if staining disappears with the blocking peptide, the signal is specific to that epitope.