Immunohistochemistry Markers Overview for Pathology Practice
Immunohistochemistry (IHC) is the cornerstone of modern diagnostic pathology. By visualising protein expression patterns, pathologists can pinpoint the lineage, differentiation state, and even the molecular drivers of a tumour. This course consolidates the most frequently tested IHC markers that appear in board‑style quizzes, with a focus on melanoma, neuroendocrine skin tumours, vascular lesions, plasma cells, Langerhans cells, and Kaposi sarcoma. Each section provides a concise explanation, practical interpretation tips, and memorable mnemonics to aid recall.
Melanocytic Markers: Spotting the Most Sensitive Indicator of Melanoma
Why SOX10 outranks HMB‑45, S100, and MART‑1
SOX10 transcription factor is now recognised as the most sensitive IHC marker for melanoma cells. Unlike HMB‑45, which highlights premelanosomes, or MART‑1 (Melan‑A) that stains cytoplasmic granules, SOX10 is a nuclear protein expressed throughout the entire melanocytic lineage, from early precursors to fully differentiated tumour cells. This broad expression translates into a higher detection rate, especially in desmoplastic and spindle‑cell melanomas where other markers may be weak or absent.
- Sensitivity: >95 % for all melanoma subtypes.
- Specificity: High, but note that peripheral nerve sheath tumours can also be SOX10‑positive; correlation with morphology is essential.
- Practical tip: Use a nuclear stain (brown DAB) and look for uniform nuclear positivity in tumour cells; a patchy or cytoplasmic pattern suggests a non‑melanocytic lesion.
Memory trick: Think of SOX10 as the “sun‑shine” of the melanoma world – it lights up the nucleus of every melanocyte, just as the sun lights up every corner of a landscape.
When to Add HMB‑45, S100, or MART‑1
Although SOX10 is the most sensitive, the other markers still have a role in a panel:
- HMB‑45 – highlights active melanosome production; useful for confirming melanin synthesis in pigmented lesions.
- S100 protein – extremely sensitive but less specific; also stains schwannian and dendritic cells.
- MART‑1 (Melan‑A) – cytoplasmic, helpful when SOX10 is equivocal, especially in metastatic deposits.
Neuroendocrine Skin Tumours: CK20 Perinuclear Dot Staining and Merkel Cell Carcinoma
Decoding the CK20 perinuclear dot pattern
CK20 is a low‑molecular‑weight cytokeratin that, in Merkel cell carcinoma (MCC), produces a characteristic “perinuclear dot” staining. Imagine tiny Polaroid stickers placed just around the nucleus – this pattern is virtually pathognomonic for MCC and is rarely seen in other tumours.
- Key feature: One to three discrete dots adjacent to the nuclear membrane.
- Diagnostic value: Helps separate MCC from small‑cell lung carcinoma (SCLC), which is typically CK20‑negative.
- Mnemonic: "CK20’s dot‑matrix = Merkel’s signature".
Differentiating Merkel Cell Carcinoma from Small Cell Lung Carcinoma
The most reliable immunostain for this distinction is TTF‑1 (thyroid transcription factor‑1). TTF‑1 is expressed in the majority of pulmonary small‑cell carcinomas but is absent in MCC.
- TTF‑1 positivity → favors SCLC.
- TTF‑1 negativity + CK20 perinuclear dots → supports MCC.
- Additional markers: Synaptophysin and chromogranin are positive in both, so they cannot be used alone.
In practice, a panel of CK20, TTF‑1, and neuroendocrine markers (synaptophysin, chromogranin) provides a robust algorithm for accurate classification.
Vascular and Endothelial Markers: CD31 and ERG
Recognising vascular lineage with CD31 and ERG
CD31 (PECAM‑1) and ERG (ETS‑related gene) are the gold‑standard markers for endothelial differentiation. CD31 is a surface adhesion molecule expressed on the luminal surface of blood vessels, while ERG is a nuclear transcription factor that drives endothelial gene programs.
- CD31: Strong membranous staining in endothelial cells; also seen in some histiocytes but with a distinct pattern.
- ERG: Uniform nuclear positivity; highly specific for vascular tumours and useful when CD31 is equivocal.
- Clinical relevance: Confirms diagnoses such as angiosarcoma, hemangioma, and epithelioid vascular tumours.
Analogy: CD31 is the “ID badge” that tells you a cell belongs to the blood‑vessel crew, while ERG is the “blueprint” that shows the cell knows how to build vessels.
Plasma Cell Identification: CD138 (Syndecan‑1)
When a biopsy shows a dense infiltrate of plasma cells, the most specific IHC marker is CD138. This transmembrane proteoglycan is expressed almost exclusively on mature plasma cells and a few epithelial cells.
- Positive staining: Strong membranous pattern in plasma cells; helps differentiate from lymphoplasmacytic infiltrates where CD20 (B‑cell) or CD3 (T‑cell) dominate.
- Diagnostic scenarios: Multiple myeloma, plasmacytoma, IgG4‑related disease, and chronic inflammatory conditions with plasma‑cell predominance.
- Mnemonic: "CD138 is the ‘uniform’ only plasma cells wear".
Langerhans Cell Identification: The CD1a‑Langerin (CD207)‑S100 Trio
Langerhans cells are specialised dendritic cells of the skin and mucosa. The most reliable immunophenotype combines CD1a, langerin (CD207), and S100 protein.
- CD1a: Surface glycoprotein that presents lipid antigens; strong membranous staining.
- Langerin (CD207): Cytoplasmic granules that form Birbeck granules; highly specific.
- S100: Nuclear and cytoplasmic positivity adds sensitivity.
- Clinical use: Diagnosis of Langerhans cell histiocytosis, distinguishing it from other histiocytic disorders.
Kaposi Sarcoma: Expected Positive and Negative Markers
Kaposi sarcoma (KS) is a vascular tumour driven by human herpesvirus‑8 (HHV‑8). The typical IHC profile includes:
- HHV‑8 LNA‑1: Nuclear positivity – the hallmark of KS.
- CD34 and CD31: Strong endothelial staining, confirming vascular origin.
- TTF‑1: Negative. TTF‑1 is a lung/thyroid marker and does not appear in KS, making it a useful exclusionary stain.
Visual cue: Think of KS as a “skin‑based vascular party” that never invites the “lung‑only” guest TTF‑1.
Practical Tips, Mnemonics, and Common Pitfalls for Residents
- Build a panel, not a single test. Relying on one marker can mislead; combine sensitivity (e.g., SOX10) with specificity (e.g., HMB‑45, MART‑1) for melanocytic lesions.
- Remember localisation. Nuclear stains (SOX10, ERG, TTF‑1) are easier to interpret than cytoplasmic or membranous patterns, especially in small biopsies.
- Use memory tricks. "CK20 dot = Merkel; TTF‑1 = lung; CD31/ERG = vascular; CD138 = plasma uniform".
- Avoid cross‑reactivity traps. S100 lights up many cell types; always correlate with morphology and additional markers.
- Check the clinical context. HHV‑8 positivity is only meaningful when the patient has risk factors for KS (e.g., HIV, immunosuppression).
Conclusion: Integrating IHC Knowledge into Daily Diagnostics
Mastering immunohistochemistry markers is a blend of memorisation, pattern recognition, and clinical reasoning. By understanding why SOX10 is the most sensitive melanoma marker, how CK20 perinuclear dots point to Merkel cell carcinoma, and why CD31/ERG confirm vascular lineage, you can construct logical diagnostic algorithms that reduce errors and improve patient care. Keep the mnemonics handy, always interpret stains in the context of morphology, and remember that a well‑designed panel is the safest path to an accurate diagnosis.
