Enzyme immunoassays (EIA) are used to visualize and quantify antigens. They use an antibody conjugated to an enzyme to bind the antigen, and the enzyme converts a substrate into an observable end product. The substrate may be either a chromogen or a fluorogen. Immunostaining is an EIA technique for visualizing cells in a tissue (immunohistochemistry) or examining intracellular structures (immunocytochemistry). Direct ELISA is used to quantify an antigen in solution.