Which enzyme and condition is used for intracellular antigen exposure at 37°C for 20 minutes?

• A. Proteinase K 20 µg/mL 37°C (20 min)
• B. Trypsin 0.05% to 0.1% 37°C (10 to 40 min)
• C. Pepsin 0.40% 37°C (30 to 180 min)
• D. Bromelin 37°C (15 min)

Answer: (A)

Unmasking intracellular epitopes often relies on enzymatic antigen retrieval, where controlled proteolysis breaks the cross-links created during fixation to reveal hidden antigen sites without destroying tissue structure. Proteinase K at 20 µg/mL used at 37°C for 20 minutes is a classic, moderate digestion that efficiently exposes intracellular antigens while preserving morphology. This specific combination is well balanced: enough enzyme activity to reveal epitopes but not so harsh that the tissue degrades.

Other enzymes can work for retrieval but require different conditions. Trypsin at 0.05–0.1% at 37°C for 10–40 minutes is more variable and can be harsher for some tissues. Pepsin at 0.40% at 37°C for 30–180 minutes relies on acidic conditions and longer exposure, which may alter morphology or epitope integrity. Bromelain is less commonly used and not typically aligned with this exact parameter set. Therefore, the Proteinase K option best fits the requested condition.