Sodium Citrate Buffer, 0.01 M, pH 6.0

Polymer-based two-step IHC detects tissue antigens by first binding an unlabeled primary antibody to the antigen and then applying an HRP-polymer secondary reagent that carries multiple secondary antibodies and HRP molecules on a polymer backbone; the localized HRP converts chromogens such as DAB or AEC into visible deposits for light-microscopic interpretation. The method is \"two-step\" because the primary antibody step is followed directly by the polymer-enzyme secondary reagent, rather than by separate secondary-antibody and avidin-biotin complex steps; published comparisons reported similar or higher sensitivity than several multistep systems and avoidance of endogenous-biotin interference.

Immunohistochemistry-Frozen (IHC-F) of frozen samples is a widely used technique for detecting and locating specific antigens within preserved cellular structures. Unlike formalin-fixed paraffin-embedded samples, frozen tissues retain their natural antigenicity, making them particularly suitable for targets sensitive to chemical fixation, and the procedure is relatively simple and rapid.

Avidin-biotin or streptavidin-biotin immunohistochemistry detects tissue antigens by binding a primary antibody to the target antigen, then detecting that antibody with a biotinylated antibody and an avidin-biotin-enzyme or streptavidin-enzyme detection complex; the enzyme reaction produces a visible chromogenic deposit at the antigen site for light-microscopic localization. The classic ABC method uses the high-affinity avidin-biotin interaction to bridge biotinylated secondary antibody and biotinylated peroxidase, and early comparative studies reported stronger immunoperoxidase staining than PAP-based methods in formalin-fixed tissue sections.

Double or multiplex chromogenic IHC detects two or more protein targets in the same FFPE tissue section by repeated antigen-antibody binding, enzyme-linked detection, chromogen deposition, image capture, and, for higher-plex workflows, removal or destaining before the next staining cycle. Chromogenic readouts are generated as colored precipitates at antigen sites, enabling evaluation of marker expression, cell phenotype, and spatial relationships in preserved tissue architecture. Classic examples include MICSSS, which performs iterative chromogenic IHC staining, scanning, and destaining on a single slide, and p16/Ki-67 dual staining, which uses chromogenic co-detection to identify cervical cells with combined cell-cycle deregulation and proliferation signals.