Avidin–Biotin–Peroxidase–Antiperoxidase (ABPAP) Complex: An Immunocytochemical Method with Enhanced Sensitivity

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Abstract
The avidin–biotin–peroxidase–antiperoxidase complex method (ABPAP) of immunohistochemistry employs the sequential application of the peroxidase–antiperoxidase complex (PAP) and the avidin–biotin–peroxidase complex (ABC) technics. To assess the efficacy of ABPAP in the detection of cellular antigens and to compare its utility with that of protease digestion technics, the authors studied cytokeratin (CK) reactivity in six examples each of colon, prostate, and breast cancer and Leu-M1 reactivity in five ductal breast carcinomas. ABC, PAP and ABPAP were applied to these cases alone and in combination with prior pepsin digestion of deparaffinized sections. With respect to the number of cells that stained and their intensity in each case, there was significant enhancement of CK and Leu-M1 reactivity with ABPAP, as compared with ABC and PAP when pepsin digestion was not employed. Further, ABPAP provided undigested CK staining results comparable to those seen with the use of ABC or PAP with pepsin digestion. Protease treatment was uniformly detrimental to the visualization of Leu-M1, regardless of the method employed. In summary, ABPAP was clearly superior to ABC and PAP in all settings analyzed. It may prove to be a useful adjunct in the immunohistochemical visualization of tissue antigens, when more routine technics yield suboptimal results.