Immunocytochemistry

Immunocytochemistry is a fundamental lab practice which uses labelled antibodies that target antigenic receptors on the cell membranes to determine the presence of certain diseases (e.g., cancers). It differs from immunohistochemistry in that immunocytochemistry is performed on samples of intact cells that have had most, if not all, their surrounding extracellular matrix removed. This includes cells grown within a culture, deposited from suspension, or taken from a smear. In contrast, immunohistochemical samples are sections of tissue, where each cell is surrounded by tissue architecture and other cells normally found in the intact tissue. There are many methods to perform immunological stains on tissues, including the direct method of staining. One method involves the use of a fluorescent or colored dye bonded directly to the antibody that is then allowed to bind to the antigen (marker) on a cell. Alternatively, there are many indirect methods. In one such method, the antigen is reacted with a primary antibody which binds directly to the antigen, followed by a secondary antibody which binds to the primary antibody. Next, a tertiary reagent containing an enzymatic moiety is applied and binds to the secondary antibody. When the quarternary reagent, or substrate, is applied, the enzymatic end of the tertiary reagent converts the substrate into a pigment and another product, which stains the cell, usually a reddish brown color. Some examples of substrates used (also known as chromogens) are AEC (3-Amino, 9-Ethyl Carbazole), or DAB (Diaminobenzidine). Presence of one of these reagents after application of the chromogen represents a positive stain. Immunocytochemical stains can be used when an H&E (Hematoxylin and Eosin) diagnosis cannot be made or to provide additional predictive information regarding treatment (in some cancers, for example).