Eosin is a
fluorescent acidic / negative compound that binds to and forms salts with basic, or eosinophilic, compounds containing positive charges (such as proteins that are basic / positive due to the presence of amino acid residues such as
arginine and
lysine) and stains them dark red or pink as a result of the actions of
bromine on
fluorescein. In addition to staining proteins in the
cytoplasm, it can be used to stain
collagen and muscle fibers for examination under the
microscope. Structures that stain readily with eosin are termed
eosinophilic.