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Immunohistochemistry — principles, panels and predictive markers
Immunohistochemistry detects a protein in tissue with a specific antibody and a chromogen (usually brown), and serves three purposes: determining tumour lineage, suggesting the organ of origin in a metastasis, and measuring markers that predict response to treatment — always in a panel and with controls, not by a single marker.
Why it matters in the exam
The bank asks which marker suits which tumour and how predictive markers are read. In an undifferentiated tumour, a first panel separates carcinoma (keratins), lymphoma (CD45), melanoma (S100, SOX10) and sarcoma (by type: desmin and myogenin for muscle, CD31 and ERG for vessels). Neuroendocrine markers: synaptophysin, chromogranin and INSM1. In a metastasis of unknown origin, the CK7 and CK20 combination guides: CK7-positive and CK20-negative in lung and breast, CK7-negative and CK20-positive in colon, and both positive in the urinary tract and pancreas.
Organ markers: TTF-1 and Napsin A for lung adenocarcinoma (TTF-1 also in thyroid), CDX2 and SATB2 for intestine, GATA3 for breast and urinary tract, PAX8 for kidney, Müllerian tumours and thyroid, p40 for squamous carcinoma, NKX3.1 and PSA for prostate, and WT1 with calretinin for mesothelioma. Predictive markers in breast: ER and PR are positive from 1% stained cells; HER2 is scored 0, 1+, 2+ or 3+, where 3+ is positive, 0 and 1+ negative, and 2+ requires ISH testing. Testing of the mismatch-repair proteins — MLH1, PMS2, MSH2 and MSH6 — detects tumours with microsatellite instability.
The quality of the result depends on the pre-staining stage: fixation in 10% neutral buffered formalin, short cold ischaemia time and suitable fixation duration. Positive and negative controls and an internal control in the tissue itself are essential. "Aberrant" marker expression exists — for example keratin in some sarcomas — so a diagnosis is not made on one marker. PECAM-1 (CD31), the molecule of leukocyte migration through endothelium, is also used as a vascular marker.
Key points
- Immunohistochemistry for lineage, origin and treatment.
- CK7 and CK20 guide the origin of a metastasis.
- TTF-1 for lung and thyroid; CDX2 for intestine; GATA3 for breast and urinary; PAX8 for kidney and Müllerian.
- ER positive from 1%; HER2 2+ requires ISH.
- Correct fixation and controls determine reliability.
Summary table
| Purpose | Markers | Example | Pitfall |
|---|---|---|---|
| Lineage | Keratins, CD45, S100 and SOX10, desmin, CD31 and ERG | Carcinoma versus lymphoma versus melanoma | Relying on vimentin |
| Origin of metastasis | CK7 and CK20, TTF-1, CDX2, GATA3, PAX8 | CK7-negative and CK20-positive — colon | Diagnosis by one marker |
| Neuroendocrine | Synaptophysin, chromogranin, INSM1 | Carcinoid, small cell lung cancer | Forgetting Ki-67 for grade |
| Predictive in breast | ER, PR, HER2 | ER positive from 1%; HER2 2+ → ISH | Reading 2+ as positive |
| Mismatch repair | MLH1, PMS2, MSH2, MSH6 | Loss — microsatellite instability | Interpreting without an internal control |
Free exam questions
אישה בת 68 מגיעה אל הגניקולוג עם תלונות של גרד כרוני ופלאקים אדומים בעריה ) Vulva ( מזה כשנתיים, אשר אינם מגיבים למשחות סטרואידים מקומיות. בביופסיה נראים תאים גדולים עם ציטופלסמה בהירה וגרעינונים בולטים, מפוזרים כתאים בודדים ובקבוצות קטנות באפידרמיס. תאים אלה חיוביים ל- CK7 ול- Mucicarmine ושליליים ל- S100 . מהי האבחנה הסבירה ביותר? (the question as examined, in Hebrew)
- א. Extramammary Paget disease of the vulva
- ב. Vulvar melanoma
- ג. ) High-grade squamous intra-epithelial lesion (HGSIL
- ד. Langerhans cell histiocytosis
איזה מסלול איתות עיקרי מעורב בגידול בגוף הלבלב מסוג אדנוקרצינומה , שבצביעות פתולוגיה חיובי ל- CK7 ,חיובי ושלילי ל- CDX2 ו - CK20 ? (the question as examined, in Hebrew)
- א. PI3K/AKT/mTOR
- ב. KRAS/MAPK
- ג. SMAD4/TGFβ
- ד. Wnt/β-catenin
In the full version for subscribers
- A simple algorithm
- Memory aid
- Exam traps
- 5 review cards
- Enough for IMLE
The full version is open to subscribers of: Diagnostic pathology, Oncology, Medical licensing exam (IMLE).
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Sources
- Dabbs DJ. Diagnostic Immunohistochemistry: Theranostic and Genomic Applications, 6th edition (2022) — Principles, controls and diagnostic panels
- ASCO/CAP Guideline: Human Epidermal Growth Factor Receptor 2 Testing in Breast Cancer, 2023 update — Scoring and reflex testing
- ASCO/CAP Guideline: Estrogen and Progesterone Receptor Testing in Breast Cancer, 2020 — Positivity thresholds and pre-analytic requirements
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.