Often, when a specific type of cancer is diagnosed, it rules out other types of cancers in the process. This is so true in cases of mesothelioma, in which the most usual "differential diagnosis" is mesothelioma versus adenocarcinoma.
During the process of biopsy, the surgeon takes out some tissue samples to be sent to a laboratory where slides are made for viewing and analysis of a pathologist. Along with the specimen tissues is a request form where the details about the patient's medical history is recorded as well as a description of the area in the body where the tissues were taken from. Each specimen is numbered in order to identify it with the particular patient. The pathologist will then perform a thorough examination by first describing the tissue, then placing it inside a cassette which is kept in a fixative that permanently preserves the tissue. Once fixed, it is made into a paraffin block where pathologists slice off thin microscopic pieces which will be stained to identify the patient's diagnosis.
In definition, immunohistochemistry is "a technique of identification and analysis of cell types according to the binding of antibodies to certain parts of the cell". This process helps diagnose mesothelioma against adenocarcinoma and other types of cancer.
Before, pathologists used "negative markers" that are expressed in adenocarcinoma but not in mesothelioma and helped distinguish the former from the latter. Pathologists had to deal with the absence, rather than presence of certain markers; and this has made it harder to confirm a diagnosis. Some markers are positive in adenocarcinoma diagnosis but negative in mesothelioma; such as CD 15 (LeuM1), CEA (carcinoembryonic antigen), Bg8 (epithelial glycoprotein), BerEp4 (tumor glycoprotein) and MOC-31 (tumor glycoprotein).
In recent years, what came to the forefront were "positive markers" manifested by mesothelioma. These markers are normally negative in adenocarcinoma; these are cytokeratin 5, calretinin, HBME-1, N-cadherin, mesothelin, vimentin, thrombomodulin and WT-1 (Wilm's tumor gene product).
While the markers are popularly used to help in diagnosing mesothelioma's epithelial sub-type, it is important to note that they may also be manifested in other kinds of cancers; they may not essentially apply to the sarcomatoid or bi-phasic sub-types of mesothelioma. Your physician can always recommend a more advanced laboratory if he thinks your diagnosis is inconclusive in any way.