Immunogold labeling is a staining technique used in electron microscopy. This method follows the same patterns of the Indirect immunofluorescence. Colloidal gold particles are most often attached to secondary antibodies which are in turn attached to primary antibodies designed to bind a specific antigen or other cell component. Gold is used for its high electron density which increases electron scatter to give high contrast 'dark spots'. The labeling technique can be adapted to distinguish multiple objects by using differently-sized gold particles.
J.J.Bozzola and L.D. Russell Eletron Microscopy: Principles and Techniques for Biologist. (Ed.) 1999 Jones and Barlett Publishers