When a tumor lacks definitive morphological features on routine H&E, immunohistochemistry identifies its lineage, primary site, and subtype through characteristic patterns of protein expression. IHC has transformed the diagnosis of undifferentiated malignancies and is central to modern surgical pathology.
Carcinoma vs. Sarcoma vs. Lymphoma
The most common IHC application is distinguishing broad tumor categories. Carcinomas (epithelial tumors) express cytokeratins (CK) and epithelial membrane antigen (EMA). Sarcomas (mesenchymal tumors) express vimentin, and often desmin (muscle), CD31/CD34 (vascular), or S100 (neural). Lymphomas express CD45 (leukocyte common antigen) and lineage-specific markers (CD20 for B-cell, CD3 for T-cell). Melanomas express S100, HMB45, Melan-A, and SOX10.
A typical initial panel for a poorly differentiated malignancy includes cytokeratin (AE1/AE3), vimentin, CD45, and S100. The pattern of positivity usually narrows the differential: cytokeratin-positive, vimentin-negative favors carcinoma; vimentin-positive, cytokeratin-negative favors sarcoma; CD45-positive favors lymphoma.
Determining Primary Site
For metastatic carcinomas with an unknown primary, IHC panels identify the likely tissue of origin using lineage-specific markers. CK7 and CK20 are the most widely used pairing: CK7+/CK20− suggests lung, breast, thyroid, or endometrial primary; CK7−/CK20+ suggests colorectal or Merkel cell; CK7+/CK20+ suggests bladder, ovarian mucinous, or pancreatic; CK7−/CK20− suggests prostate, renal, or liver.
Organ-specific markers include TTF1 (lung, thyroid), PAX8 (thyroid, renal, ovarian, endometrial), GATA3 (breast, bladder), PSA (prostate), HepPar1 (liver), CDX2 (intestinal), and ER (breast, endometrial, ovarian). Careful panel selection based on clinical context (patient sex, imaging findings, serum tumor markers) maximizes diagnostic yield.
Neuroendocrine Tumors
Neuroendocrine tumors (NETs) are identified by expression of chromogranin A, synaptophysin, and CD56. Ki-67 proliferation index is used for grading: G1 (<3%), G2 (3-20%), G3 (>20%). Site-specific markers include TTF1 (lung), ISL1 (pancreas), and SATB2 (lower gastrointestinal tract).
Lymphoma Subtyping
Lymphoma classification relies on an extensive panel of cluster of differentiation (CD) markers. Diffuse large B-cell lymphoma (DLBCL) expresses CD20, CD79a, PAX5, and BCL6; subclassification by cell of origin (germinal center vs. activated B-cell) requires CD10, BCL6, and MUM1. Hodgkin lymphoma shows CD30 and CD15 positivity with CD45 negativity. Anaplastic large cell lymphoma (ALCL) expresses CD30 and ALK. IHC controls are essential for lymphoma panels due to the large number of antibodies used.
Sarcoma Subtyping
Soft tissue tumors are classified by their line of differentiation. Gastrointestinal stromal tumor (GIST) expresses DOG1, CD117 (c-KIT), and often CD34. Synovial sarcoma shows cytokeratin and EMA positivity with TLE1 nuclear expression. Ewing sarcoma expresses CD99 and FLI1. Rhabdomyosarcoma expresses desmin, MyoD1, and myogenin. Many sarcomas carry defining translocations that can be confirmed by FISH or PCR.
Pitfalls and Interpretation
IHC interpretation requires awareness of potential pitfalls: aberrant expression (cytokeratin-positive sarcomas, vimentin-positive carcinomas), loss of expected markers in poorly differentiated tumors, cross-reactivity (S100 in many non-melanocytic tumors), and fixation effects that may reduce sensitivity. Predictive IHC markers such as ER, HER2, and PD-L1 have specific scoring systems and require validated, standardized protocols.
