Hypothesis for the mechanism for heat-induced antigen retrieval occurring on fresh frozen sections without formalin-fixation in immunohistochemistry.

The mechanism involved in heat-induced antigen retrieval (AR) remains unproven but probably utilizes the breaking of formalin-induced cross-linkages. We investigated the effectiveness of heat-induced AR on immunohistochemistry and dot-blot analysis using rat uterus tissue sections and protein extracts without formalin-fixation. The unfixed frozen sections, which did not show immunostaining with nine antibodies, were clearly stained after heating the sections. In the dot-blot analysis, the immunoblot sensitivity of detection was greatly enhanced by heating the protein-blotted membrane. These results indicate that other mechanisms of breaking formalin-induced cross-linkages may be present. We propose that one of the other mechanisms for heat-induced AR is that accessibility to the target epitopes of antigenic proteins is limited by natural steric barriers even in the fresh state caused by the antigenic protein itself.