It is well known that certain stains and dyes specifically combine with certain chemical components of the cell. Cytochemical techniques take advantage of this fact and help in localisation and identification of various constituents like proteins, nucleic acids, lipids, etc. within a cell. For this a particular molecule is first immobilized and then identified by its reaction with a stain. Some molecules are also identified by their physical properties like absorptian of radiation at a particular wavelength. For example, the nucleic acids absorb UV rays at 260 nm whereas the proteins absorb at 280 nm.
The amount of the chemical component present in a cell can be quantified provided the reaction betwzen the molecule present and the stain is strictly on 1: 1 basis, as in the case of Feulgen reaction for DNA explained.
A large number of dyes are in use to stain different chemical components of the cell.However, the chemical basis for the specificity of many dyes is not known, so only qualitative information can be obtained.