**Abstract**

Heat-induced antigen retrieval (HIAR) method reported by Shi et al. in 1991 has greatly contributed not only to immunohistochemistry but also to studying gene expressions using archived formalin-fixed and paraffin-embedded (FFPE) specimens. Heating cleaves crosslinks (methylene bridges) in formaldehyde-fixed proteins and extends polypeptides to expose epitopes hidden in the inner portion of antigens or covered by adjacent macromolecules. In this chapter, the following topics are described to reconsider the concept of immunohistochemistry flexibly and to apply HIAR for further immunological studies using a variety of specimens: (1) antigen-antibody interactions in tissues; (2) mechanisms of chemical fixation with formaldehyde, glutaraldehyde, and osmium tetroxide; (3) unmasking of epitopes using HIAR for specimens fixed with chemical fixatives, including highly masked epitopes; (4) a standardized fixative for immunoelectron microscopy-based HIAR; (5) HIAR for conventionally processed electron microscopy specimens embedded in epoxy resins; and (6) effects of antibody diluents on immunohistochemistry.

**Keywords:** heat-induced antigen retrieval, antigen-antibody reaction, mechanism of chemical fixations, mechanisms of antigen retrieval, epitope exposure, immunoelectron microscopy, standardized fixative, osmicated specimens, antibody diluents
