Unmasking the mysteries of antigen or epitope retrieval and formalin fixation.

It has revolutionized the practice of diagnostic immunohistochemical analysis, permitting the use of a much wider range of monoclonal and polyclonal antibodies on deparaffinized, formalin-fixed tissue samples, and it has profoundly increased the sensitivity of many other antibodies, dramatically altering our previous notions of selected antibody sensitivity and specificity. Yet while we immunohistochemists use it all the time, our knowledge of why or how it works is rudimentary at best and probably wrong at worst. Of course, the subject is antigen (or epitope) retrieval, a technique introduced in the seminal publication of Shi and colleagues 13 years ago. 1 Somewhat ironically, after railing for years against the dangers of excessive heat, eg, during tissue processing or slide drying, immunohistochemists started advocating the application of 100°C heat to enhance—and even permit—immunohistochemical staining in the context of deparaffinized, formalin-fixed tissues. During the past decade and a half, antigen- and epitoperetrieval techniques have had such a profound effect on diagnostic immunohistochemical analysis that the published literature must be divided into the “pre–antigen-epitope retrieval” and “post–antigen-epitope retrieval” eras, with the dividing line in the early 1990s. Indeed, reported antibody sensitivity and specificity results as published in the pre–antigen-epitope retrieval era should be viewed with great caution and skepticism. While Shi and coworkers 1 initially speculated that the