PIER, also known as enzyme retrieval, was the first method developed to reduce the antigen masking effects of formalin fixation. With PIER, proteolytic enzymes are used to break apart the protein cross-linkages formed by formalin fixation. Proteolytic enzymes used as part of PIER include Proteinase K, Pepsin, and Trypsin.
Caution must be given to temperature and time when using proteolytic-induced epitope retrieval methods. Each enzyme activates at a specific temperature and variances in time and/or temperature can cause extensive damage to the tissue section very quickly. This could result in loss of staining signal or complete section loss.
PIER can be detrimental to the visualization of some antibodies, sometimes producing false-positive or false-negative results. Additionally, PIER is favored when demonstrating immunoglobulins and complement in formalin-fixed, paraffin-embedded (FFPE) samples.
