In cytology, the practical utility of IHC includes:

• A. Classification of poorly-differentiated neoplasms
• B. Differentiation of primary from metastatic tumors
• C. Determining the site of origin of metastatic lesions
• D. All of the above

Answer: (D)

Immunohistochemistry (IHC) in cytology provides protein-based clues about what type of cell the tumor originated from and where it may have come from. This makes it especially helpful when morphology alone can’t distinguish what you’re looking at.

First, it helps classify poorly differentiated neoplasms. When tumor cells lose their distinctive features, IHC markers reveal lineage and differentiation—epithelial, mesenchymal, neuroendocrine, or other identities—and guide you toward a more accurate classification.

Second, it aids in telling a primary tumor from a metastasis. The marker profile often aligns with a particular organ or tissue type, helping you decide whether a lesion in an organ is a new primary tumor or a spread from somewhere else.

Third, it assists in determining the site of origin of metastatic lesions. By using panels of organ-specific or pattern-associated markers, you can narrow down the likely source of metastatic cells, even when the primary site isn’t obvious clinically.

Because it supports all three roles—classification of poorly differentiated tumors, distinguishing primary from metastatic lesions, and identifying the likely site of origin of metastases—immunohistochemistry is broadly useful in cytology.