Questions
2 questions per theory paper
Difficulty
Medium
Importance
High yield for medical/BMLT university exams
Overview
Special stains and Immunohistochemistry (IHC) are critical diagnostic techniques used in histopathology to identify specific tissue components or cellular proteins. Understanding these processes is vital for final-year students as they bridge the gap between routine H&E staining and precise molecular pathology, frequently appearing in both theory and practical viva examinations.
Principles of Special Stains
Special stains, or histochemical stains, are used when H&E fails to demonstrate specific substances like carbohydrates, lipids, or pigments. They rely on chemical reactions, such as oxidation or reduction, to selectively bind to target tissue elements.
- PAS stain: Detects glycogen and mucins (magenta color)
- Ziehl-Neelsen: Identifies acid-fast bacilli (bright red)
- Masson's Trichrome: Differentiates collagen from smooth muscle
- Prussian Blue: Stains hemosiderin deposits blue
- Grocott's Methenamine Silver (GMS): Used for fungal walls
Principles of Immunohistochemistry (IHC)
IHC is a laboratory method that uses the principle of antigen-antibody binding to detect specific protein markers in cells of a tissue section. The process involves an antigen-antibody reaction followed by visualization using a chromogen or fluorescent label.
- Direct method: Primary antibody labeled directly with a tag
- Indirect method: Unlabeled primary antibody + labeled secondary antibody
- Enzymatic detection: Peroxidase/alkaline phosphatase systems
- Chromogen: DAB (3,3'-Diaminobenzidine) is the most common brown precipitate
- Antigen retrieval: Heat-induced epitope retrieval (HIER) is mandatory for FFPE tissue
Diagnostic Applications
These techniques are essential for tumor classification, determining tissue origin, and identifying infectious agents. Clinical decisions regarding targeted cancer therapy often depend entirely on the expression profile revealed by IHC.
- ER/PR/HER2: Breast cancer prognosis and treatment
- Cytokeratin (CK): Epithelial marker
- Vimentin: Mesenchymal marker
- CD20/CD3: Lymphoma subtyping
- S100: Melanoma diagnosis
Formula Sheet
Specificity = True Negatives / (False Positives + True Negatives)
Sensitivity = True Positives / (True Positives + False Negatives)
Exam Tip
Always link the stain/antibody to its clinical application, such as 'CD20 for B-cell lymphomas', to secure full marks in long-answer questions.
Common Mistakes
- Confusing the purpose of histochemical stains (chemical reaction) with IHC (antigen-antibody binding).
- Failing to mention antigen retrieval steps when asked about IHC protocols on formalin-fixed tissue.
- Incorrectly identifying the standard chromogen color (DAB is brown, not blue or red).
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