Microscopic and Immunohistochemical Diagnosis
Core Concepts
IHC is a tool for lineage confirmation, not a substitute for morphology. Understand antibody panels by lineage: epithelial (CK, p63, p40), mesenchymal (vimentin, SMA, desmin), neural (S100, SOX10), vascular (CD31, CD34, ERG), lymphoid (CD3, CD20, CD5, CD10, BCL2, cyclin D1), and melanocytic (S100, SOX10, Melan-A). Know the principles of antigen retrieval, fixation, and controls.
Common Traps
- Treating a single positive marker as diagnostic without morphology.
- Ignoring fixation artifacts and edge effects that cause false-negative staining.
- Misinterpreting cytoplasmic vs nuclear staining patterns (e.g., p63 nuclear, CK cytoplasmic).
- Forgetting that SOX10 is positive in melanoma and schwannoma but negative in most carcinomas.
Rule of Thumb
Use a "lineage first, then subtype" strategy. A worked example: a spindle cell lesion in the neck shows S100 positivity and SOX10 positivity - think neural or melanocytic; add Melan-A and HMB-45 to separate melanoma from schwannoma. If cytokeratin is also positive, consider synovial sarcoma or a sarcomatoid carcinoma.
Quality Control
Include positive and negative controls with every run and document antibody clones and lot numbers.