Mesothelioma Immunohistochemistry Immunohistochemistry (IHC) has become an invaluable tool in the differentiation of histological mesothelioma subtypes with the use of antigens, which are substances that trigger the production of antibodies by the immune system. By locating distinct protein antigen markers within mesothelioma tumor cells, a diagnosis can be made that is far more accurate than diagnoses using imaging technologies alone. With mesothelioma patients, IHC is most often used to differentiate the disease from others that sometimes mimic it. Antigens and antibodies (immunoglobulins) bind specifically in all living cells. IHC takes advantage of this property to identify specific types of cells within a sample. A tissue biopsy is obtained from the individual who is being diagnosed. The sample is then sliced into extremely thin sections—preparations as little as one cell thick are not uncommon. IHC can be performed on slide preparations or in free-floating samples, although the former procedure is more common. IHC can also be performed on either fresh or frozen tissue. The lab technician introduces specialized antibodies into the specimen that have been “labeled” in a particular way, generally through the introduction of a stain or fluorescent dye, allowing the definitive identification of the antigen proteins associated with those antibodies. The sample is then washed so that the markers remain only in the tagged cells. Direct IHC Direct IHC involves a straightforward reaction between the antigen and the labeled antibody, whereas indirect IHC involves the introduction of two antibodies—a primary unlabeled antibody and a secondary labeled antibody. Indirect IHC has higher sensitivity and is far more commonly used than direct IHC. Cellular events associated with the tumor—for example, an increase in cell death—will be highlighted by the stained tissue sample. The pathologist or other diagnostician who examines the prepared slides will make his or her determination based on those cellular events. Protein Markers The two most commonly used protein markers in the diagnosis of mesothelioma histologic subtypes are BerEP4 and vimentin. BerEP4 is a monoclonal antibody that can be used reliably to label epithelial cells but is not reactive with mesothelial cells. While it has not so far proven to be a useful diagnostic determinant to indicate whether a patient is suffering from mesothelioma, it has been extremely useful in the differential diagnosis of mesothelioma versus adenocarcinoma or other similar types of cancer. Vimentin is a filament protein found in microvilli. Like BerEP4, it has been found to be a useful IHC marker in differentiating malignant mesotheliomas from adenocarcinomas, but it cannot reliably distinguish malignant from benign hyperplastic mesothelial cells.