This chapter outlines the biologic background and the tissue distribution of the most significant immunohistochemical markers for soft tissue tumors. Tumor type-specific applications are detailed in Chapters 6 to 35. Markers for mesothelioma are presented in Chapter 28, those for metastatic carcinomas in Chapter 29, and those for lymphohematopoietic tumors in Chapter 35. Some very specialized “one-tumor markers” are discussed only with their specific applications in tumor-specific Chapters 6 to 35. Several reviews are suggested for further reading.

The most important cell type markers are grouped under subheadings of endothelial and multispecific, muscle cell, neural and neuroendocrine, melanoma and histiocytic markers, keratins, other epithelial and mesothelial markers, other cell type markers, and cell cycle markers. These markers and their main diagnostic targets have been listed in Table 3.1. Conversely, several previously used immunohistochemical markers have proved either diagnostically nonspecific or impractical to use and have been replaced by better markers (Table 3.2).

Immunohistochemistry is the most practical way to evaluate the presence of certain protein and carbohydrate epitopes on tissue sections. Evaluation of cell- or tumor-type specific or cell cycle related markers is often highly significant, both diagnostically and differentially, although very few markers are totally specific for one tumor type, and no cell cycle marker can separate benign and malignant tumors.

Detailed knowledge of antigen expression in normal tissues and different tumors is also important for understanding the diagnostic potential of various markers. There are estimated 30,000 to 50,000 genes, some of which undiscovered as of yet.