Immunocytochemistry, based on the specific interaction between antigens and antibodies, has been widely applied in recent decades to studies of the localization of specific proteins in cells. In this technique, cells (or tissue sections, in immunohistochemistry) to be studied are incubated in a solution containing an antibody to the protein of interest. If the protein is present in the cells and the incubation conditions are suitable, the antibody binds specifically to the targeted protein. Subsequently, its location can be detected using a light or electron microscope depending on the type of compound (fluorescent substance, enzyme or gold particles) used for antibody labeling. This immunocytochemical procedure requires the development of antibodies specific for the proteins being examined (i.e., primary antibodies), and techniques for labeling the antibodies. In fact, various techniques for producing the antibodies, both polyclonal and monoclonal,
have been developed for the specific and sensitive detection of antigens by immunocytochemistry. Two labeling methods — direct and indirect — have also been introduced for immunocytochemistry. In the direct method, primary antibodies are directly labeled with a fluorescent dye, which enables visualization of the protein of interest with a fluorescent microscope. In the indirect method, primary antibodies are unlabeled but are visualized using labeled secondary antibodies that can bind to the corresponding primary antibodies.