Immunohistochemistry (IHC): Introduction, Application, and Keynotes

Introduction to IHC

Immunohistochemistry (IHC) is a powerful laboratory technique that combines anatomical, immunological, and biochemical methods to detect specific antigens (proteins) in tissue sections. By exploiting the principle of antibodies binding specifically to antigens in biological tissues, IHC allows for the visualization of protein distribution and localization within the proper histological context.

• Core Principle: Specific antigen-antibody binding. Antibodies are used to “stain” tissue samples, which are then visualized under a light or fluorescence microscope.
• Components: Involves primary antibodies (binding the target) and secondary antibodies (labeled with an enzyme or fluorophore for detection).
• Common Method: Most commonly performed on formalin-fixed, paraffin-embedded (FFPE) tissue, allowing for permanent records and easy storage.
• Visualization: Chromogenic detection (e.g., DAB, resulting in brown staining) is most common in diagnostics, while immunofluorescence is used for multiple, simultaneous targets.

Applications of IHC

IHC is essential in clinical diagnostics and research, with key applications including:

• Cancer Diagnosis & Classification: Distinguishing benign from malignant tumors and determining tumor origin (e.g., differentiating carcinoma, melanoma, or sarcoma).
• Prognostic & Predictive Markers: Evaluating tumor aggressiveness and guiding treatment. A key example is testing for HER2 amplification in breast cancer.
• Infectious Disease Detection: Identifying pathogens like viruses, bacteria, and fungi in tissues.
• Neuropathology: Detecting proteins associated with Alzheimer’s or Parkinson’s disease.
• Basic Research: Mapping protein expression in tissues and studying cellular behavior.

Keynotes & Best Practices

• Antigen Retrieval (AR): Essential step (heat-induced or enzymatic) to break protein cross-links caused by fixation, making antigens accessible to antibodies.
• Fixation: Proper fixation (e.g., 10% neutral-buffered formalin) is critical. Under-fixation can cause poor preservation, while over-fixation can mask antigens.
• Controls: A positive control (tissue containing the antigen) and a negative control (omitting the primary antibody) must be run with each test for accuracy.
• Blocking: Used to reduce non-specific background staining, usually with serum or BSA.
• Troubleshooting: False negatives are more common than false positives and can be caused by improper antigen retrieval or over-fixation.
• Automation: Modern pathology labs often use automated methods to increase reproducibility and high-volume processing.

Further Readings

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC6749998/
2. https://www.scitechnol.com/peer-review/immunohistochemistry-ihc-principles-techniques-and-applications-z1gC.php?article_id=27668
3. https://www.researchgate.net/publication/329590333_An_Introduction_to_the_Performance_of_Immunohistochemistry_Methods_and_Protocols
4. https://my.clevelandclinic.org/health/diagnostics/25090-immunohistochemistry
5. https://www.sinobiological.com/category/ihc-introduction
6. https://www.thermofisher.com/au/en/home/life-science/protein-biology/protein-biology-learning-center/protein-biology-resource-library/pierce-protein-methods/overview-immunohistochemistry.html
7. https://www.ptglab.com/media/mjopz1mm/ihc-guide_v13_digital.pdf?srsltid=AfmBOoqKkbQ7lqMF7dCXWAGrgbgwCyAmRu0SwWimXmPIghrIYloRsTQ0
8. https://en.wikipedia.org/wiki/Immunohistochemistry
9. https://www.sciencedirect.com/science/article/pii/S0740257024000467
10. https://www.antibody-creativebiolabs.com/immunohistochemistry.htm