In immunohistochemistry, fluorescent molecules including fluorescein isothiocyanate (FITC), Texas Red, or Alexa Fluor dyes are frequently attached to primary antibodies for visualization. It is important to note that fluorescent compounds require the use of a fluorescence microscope for visualization. Additionally, enzymes like horseradish peroxidase (HRP) or alkaline phosphatase (AP) are commonly used as labels. They catalyze reactions that produce visible or fluorescent products when substrates are added. Particulate labels are also used. These are metallic substances like colloidal gold or tiny gold particles. They're primarily utilized for electron microscopic immunocytochemistry and have light-scattering properties.