Classical histological methods of investigating cellular pathology involve characterizing morphological features using light absorbing dyes and fluorescent probes. The first category of stains gives rise to different colours in different subcellular constituents due to differential binding and hence differential absorption of transmitted light. Staining was used increasingly in microscopy after Virchow's work with various pigments including those from blood (Virchow, 1847) and the most extensively used example is the combination of haemotoxylin (introduced by Waldeyer, 1863) and eosin. The former is a basophillic blue dye which binds to nuclear components and the latter is acidophillic which binds to cytoplasmic constituents. Heamotoxylin appears blue to the human eye as it absorbs red light and eosin appears yellow/orange due to blue light absorption. Without the aid of this type of differential stain combination the histopathologist would hardly exist as details of the unstained cell are essentially invisible.

Immunoperoxidase staining of specific molecules using monoclonal antibodies (Köhler and Milstein, 1975) is an extension of this type of approach with the deposition of brown/black granules which absorb light of all wavelengths at a site where the antibody binds to its target molecule. This method, of course, is used in conjunction with other stain combinations to identify the site at which th antibody binds. With the orange/blue combination of eosin plus haemotoxylin as a counterstain for the immunoperoxidase we can locate the molecule of interest as being nuclear, cell surface or cytoplasmic.