A beginner’s guide to immunohistochemistry

Using the affinity between antibodies and antigens, immunohistochemistry (IHC) is a helpful method for analysing the cellular components within tissue sections placed on glass slides. The chemistry of antibodies was not discovered until 1941, and since then, their use has increased due to the discovery of enzymes and fluorochromes conjugated to antibodies, enabling observations through optical or fluorescence microscopy (FM). Hence, immunofluorescence (IF), or IHC with fluorescent antibodies, has been associated with the advancement of FM and fluorochromes. Specifically, the discovery of three fluorochromes that emit blue, red and green led to a rise in IF application in the context of visualizing numerous components in a single cell. The proximity ligation assay (PLA) method, so termed because the antibody is conjugated with short DNA strands called PLA probes, is the most recent development in the field of antibody conjugation. Protein–protein interactions and sub-localization of proteins can both be studied using this technique. The improvement of conjugation techniques has occurred in parallel with the development of sophisticated microscopies, including confocal and slide scanners. Confocal microscopy allows for the reconstruction of three-dimensional structures while a slide scanner can digitally convert up to 100 slides' worth of high-resolution pictures. The first step in the IHC process is the collection and fixation of tissues from animal models and humans. Paraformaldehyde is the most commonly used fixative. Fixation prevents antigen blocking and creates cross-links between proteins to preserve tissue structures. Fixation is followed by cutting tissues using microtome or cryostat, then slides are treated for antigen exposition (antigen retrieval) and then included in Triton X solution, which increases membrane permeability. Subsequently, incubations with enzyme- or fluorochrome-conjugated antibodies enable observations via optical microscopy or FM, respectively. In clinical practice and research, IHC and IF are still helpful methods for identifying the antigen-proteins linked to specific diseases.