Immunohistochemical markers are monoclonal antibodies used to identify specific proteins in tissue sections. The antibody binds to the protein being assessed and a color reagent stains the protein, if in fact that protein is present in the tissue. The proteins that we search for indicate the differentiation of the tumor and help us to render a diagnosis. Identification of differentiation proteins is often required for tumors that cannot be readily classified on routine sections alone (e.g.: round cell tumors, spindle cell tumors).
We usually need to test for the presence of antibodies to the following proteins:
- Cytokeratins (a cocktail of keratin molecules of both low and high molecular weight: carcinomas and adenocarcinomas express these.
- Vimentin is an intermediate filament expressed in connective tissue tumors and melanomas
- LCA or leukocyte common antigen is found in leukemias and lymphomas
- CD20 is a protein found on B lymphocytes
- CD3 is a protein found on T lymphocytes
- CD34 is expressed in vascular tissue and certain subtypes of fibroblasts and is used to identify angiomas, angiosarcomas and specific types of fibroblastic neoplasms
- SMA or smooth muscle actin stains the actin intermediate filaments of contractile smooth muscle cells. Myofibroblastic and myoepithelial cell tumors express this marker
- Desmin is a muscle marker
- S-100 is an intermediate filament that stains neural cells as well as other cell types such as cartilage. It is employed for neural and granular cell tumors as well as melanocytic lesions
- HMB-45 stains some, but not all, melanomas