Immunohistochemical assay is performed on a skin biopsy specimen. The specimen is treated and stained with fluorescent antibodies directed against proteins of the human immune system (IgG, IgA, IgM, C3). Under a fluorescence microscope, the assay reveals the presence, pattern, and location of these antibodies or complement in the skin layers. It is the gold standard for diagnosing bullous autoimmune skin diseases such as pemphigus, bullous pemphigoid, and dermatitis herpetiformis.