A given cell type can express specific antigens. Pathologists take advantage of this property to identify the cell type or cell of origin, typically in tumors. In the IHC stains commonly used in ophthalmic pathology, a primary antibody binds to a specific antigen in or on a cell, and a secondary antibody linked to a chromogen (a compound that produces a particular color as a result of a chemical reaction) then binds to the primary antibody (Fig 3-1). The most common chromogen color product is brown (see Fig 3-1C) or red (see Fig 3-5B). A red chromogen color product is especially helpful in working with pigmented ocular tissues and with melanomas, because it contrasts with the brown melanin pigment in the uveal tissue or tumor.
Table 3-1 Checklist for Requesting an Ophthalmic Pathology Consultation
Figure 3-1 Immunohistochemistry. A, Schematic representation of the general immuno-histochemistry method. (1) The cellular antigen is recognized by the specific primary antibody; (2) a secondary antibody targeting the primary antibody is added and attaches to the primary antibody; (3) the secondary antibody reacts with the enzymatic complex, thereby activating the chromogen; (4) the final product allows visualization of the cell containing the antigen based on identification of the color product of the activated chromogen in the tissue section. B–C, A metastatic carcinoid to the orbit seen with hematoxylin-eosin (H&E) staining (B) shows bland epithelial characteristics. In C, an antibody directed against chromogranin highlights the neuroendocrine nature of the cells.
(Courtesy of Patricia Chévez-Barrios, MD.)
A specific antigen can be identified using this method, and depending on the antigen profile, the specific cell type that expresses this antigen(s) can be ascertained. Many antibodies are routinely used for diagnosis, treatment, and prognosis. The following are some common antibodies used in IHC stains; a more comprehensive list can be found in Table 3-2:
cytokeratins (many different antibodies) for diagnosis of lesions that are derived from epithelial cells (eg, adenoma, carcinoma)
desmin, myoglobin, or actin for diagnosis of lesions with smooth muscle or skeletal muscle features (eg, rhabdomyosarcoma, leiomyoma)
S-100 protein for diagnosis of lesions of neuroectodermal origin (eg, schwannoma, neurofibroma, melanoma)
Melan A and HMB-45 for diagnosis of melanocytic lesions (eg, nevus, melanoma)
chromogranin and synaptophysin for diagnosis of neuroendocrine lesions (eg, metastatic carcinoid [see Fig 3-1C], small cell carcinoma)
leukocyte common antigen for diagnosis of lesions of hematopoietic origin (eg, leukemia, lymphoma)
CD (cluster of differentiation) antigens for subtyping white blood cells, particularly lymphocytes
BAP1 for prognosis of neoplasia (eg, uveal melanoma)
Table 3-2 Common Immunohistochemical Stains in Ophthalmic Pathology
Antibodies used for IHC vary in their specificity and sensitivity, which will affect interpretation. Interpretation of IHC stains includes comparing the results of the tissue being tested with appropriate standardized controls. The specificities and sensitivities of new antibodies are continually being evaluated. In addition, automated equipment and antigen retrieval techniques are currently used to increase sensitivity and decrease turnaround time.
Excerpted from BCSC 2020-2021 series: Section 4 - Ophthalmic Pathology and Intraocular Tumors. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.