Immunohistochemistry Markers
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Direct answer
One antibody, one epitope, one colour: immunohistochemistry (IHC) turns an invisible antigen-antibody reaction brown on a slide (via chromogens like DAB) and thereby answers the three questions morphology alone cannot — what lineage is this tumour, where did it come from, and how fast is it growing. Lineage first: cytokeratins for epithelial tumours, CD45 for lymphoid, S-100 and SOX10 for neural crest and melanocytic, desmin and myogenin for muscle, CD34 and ERG for vascular, GFAP for glial. Origin next: CK7/CK20 profiles and organ-specific markers such as TTF-1 (lung and thyroid), CDX2 (gut), PAX8 (renal, thyroid, Müllerian), PSA (prostate), hepatocyte antigen and arginase-1 (liver). Behaviour last: Ki-67 labels proliferating cells and grades indolence, while receptor markers — ER, PR, HER2 in breast — are treatment decisions as much as diagnoses.
What you must remember
- CK7/CK20 grid: CK7-positive/CK20-negative — lung, breast, ovary (serous), endometrium; CK7-positive/CK20-positive — gastric, pancreatobiliary, urothelial; CK7-negative/CK20-positive — colorectal; double-negative — hepatocellular, renal, prostate, squamous carcinomas.
- Lymphoma panel logic: CD20 and PAX5 for B cells, CD3 for T cells, TdT for lymphoblasts, CD138 for plasma cells; Hodgkin Reed-Sternberg cells are CD30-positive and CD15-positive with weak PAX5; mantle cell lymphoma is CD5 and cyclin D1 positive (t(11;14)); follicular lymphoma is CD10 and BCL6 positive with BCL2 overexpression; Burkitt lymphoma is BCL2-negative with a Ki-67 near 100 per cent.
- Small round blue cell panel (paediatric and adult): CD45 (lymphoma), desmin and myogenin (rhabdomyosarcoma), CD99 and FLI1 (Ewing sarcoma family), synaptophysin and chromogranin with PHOX2B (neuroblastoma), S-100 and SOX10 (melanoma, nerve sheath).
- Site-specific anchors: TTF-1 (lung adenocarcinoma, thyroid), thyroglobulin (thyroid follicular), calcitonin (medullary thyroid), CDX2 (colorecal and intestinal), PAX8 (renal, thyroid, Müllerian), PSA and prostein (prostate), HepPar-1 and arginase-1 (hepatocellular), GATA3 (breast, urothelial), uroplakin (urothelial), CD117 (c-KIT) and DOG1 (GIST — KIT-positive in most, PDGFRA-mutant subset often DOG1-positive too).
- Mesothelioma versus adenocarcinoma: mesothelioma is calretinin, WT1 (nuclear), D2-40 and cytokeratin 5/6 positive with loss of BAP1 (or MTAP); adenocarcinoma is Ber-EP4, MOC-31, TTF-1 or PAX8 positive, BAP1 retained — a classic pairing asked as a panel.
- Germ cell and sex cord: OCT3/4, SALL4, PLAP (germ cell tumours — OCT4 for seminoma and embryonal); inhibin and calretinin (sex cord-stromal, adrenocortical).
- Proliferation and prediction: Ki-67 (MIB-1) percentage grades neuroendocrine tumours (G1 under 3 per cent, G2 to 20 per cent, G3 above) and burkitts; ER, PR and HER2 by IHC (with FISH for equivocal HER2) decide breast therapy; PD-L1 scoring guides immunotherapy in lung and urothelial cancers.
- Myeloid lineage in tissue: MPO (myeloid), CD68/CD163 (histiocytic), CD61 (megakaryocytic), CD71/glycophorin (erythroid).
Building an IHC panel logically
A 55-year-old has a supraclavicular node replaced by poorly differentiated carcinoma — metastasis of unknown primary. Step one, lineage: pan-cytokeratin positive, CD45 negative — carcinoma confirmed. Step two, the CK7/CK20 grid: CK7-positive, CK20-negative narrows to lung, breast, ovary, endometrium, thyroid. Step three, organ markers guided by sex, imaging and clinical clues: a woman gets ER, PR, GATA3, PAX8 and TTF-1; in a smoker, TTF-1 nuclear staining with granular Napsin A closes it as lung adenocarcinoma. IHC panels are branching trees, broad to specific — one first-line panel, then a confirmatory set. The same logic sorts a spindle cell tumour: vimentin-positive, then CD117 and DOG1 confirm GIST; S-100 and SOX10 with Schwannian morphology point to nerve sheath; desmin plus myogenin nuclei clinch rhabdomyosarcoma; and CD34 with ERG supports vascular origin.
Where candidates slip
Marker absolutes create most errors: TTF-1 is not lung-only (thyroid and some neuroendocrine tumours stain too); S-100 stains Schwann cells, melanocytes, cartilage, myoepithelial cells and even Langerhans histiocytes, so it is a screening, not a diagnostic, marker alone; CD117 stains GIST but also mast cells and melanoma. Second slip: calling IHC a replacement for morphology — a panel interpreted against poorly chosen morphology produces confident nonsense. Third, Ki-67 is mislabelled a "tumour marker": it marks cells in cycle, so high values mean high turnover, not necessarily malignancy (normal tonsil crypts and bone marrow proliferate briskly). Fourth, forgetting internal controls: normal tissue on the same slide must stain predictably, or the run is invalid. Fifth, nuclear versus cytoplasmic matters — WT1 must be nuclear for mesothelioma, TTF-1 nuclear for lung, beta-catenin nuclear for its specific tumour set.
Frequently asked questions
Which CK7/CK20 profile suggests a colorectal metastasis?
CK7-negative and CK20-positive, supported by CDX2 nuclear positivity — the signature that sorts a colon-primary from pancreaticobiliary and upper-gut mimics.
How does IHC separate mesothelioma from lung adenocarcinoma?
Mesothelioma: calretinin, WT1 (nuclear), D2-40, CK5/6 positive with BAP1 loss; adenocarcinoma: claudin-4, Ber-EP4 positive with organ markers such as TTF-1 or PAX8 and retained BAP1.
Which markers confirm a GIST?
CD117 (c-KIT) and DOG1 membrane positivity, with variable smooth muscle markers; KIT or PDGFRA mutation analysis follows, and succinate dehydrogenase-deficient GIST is the paediatric, syndromic exception.
What is the role of Ki-67 in reporting?
It quantifies proliferating cells, grading neuroendocrine tumours (G1-G3), separating Burkitt lymphoma (near 100 per cent) from indolent lymphomas, and flagging aggressive behaviour in many tumours.
Which IHC markers identify Hodgkin lymphoma?
Reed-Sternberg cells positive for CD30 (strong membranous and Golgi) and CD15, with weak PAX5 and CD45 negativity — the pattern that excludes non-Hodgkin lymphomas and carcinoma.