Antigen retrieval is used in immunohistochemistry (IHC) to restore epitope-antibody reactivity. 

During sample fixation in IHC, proteins undergo structural changes. This can result in protein cross-linking, which masks the target epitope and can restrict antigen-antibody binding. Antigen retrieval enhances the ability of the primary antibody to access and bind to the target protein within the tissue. 

Antigen retrieval involves using either heat-induced or enzymatic/proteolytic retrieval techniques.