Immunohistochemistry and Protein Detection Methods

Immunohistochemistry (IHC) localises and measures proteins in tissue sections using antibodies coupled to a visible label, showing where a gene product is expressed and, semi-quantitatively, how much. As the protein-level counterpart to transcript quantification, it is a routine tool in diagnostic and molecular pathology.

Definition

Immunohistochemistry is a tissue-based method that uses labelled antibodies to detect specific proteins in situ, visualising their cellular and subcellular location and providing a semi-quantitative estimate of protein expression.

Scope

This topic covers the antibody-based detection principle, the difference between qualitative localisation and semi-quantitative scoring of staining intensity and extent, the importance of standardisation and controls, and well-known applications such as hormone-receptor and HER2 assessment as illustrations of method. It treats IHC as a measurement methodology and does not provide diagnostic thresholds or treatment direction.

Core questions

• How do labelled antibodies localise a protein within a tissue section?
• How is staining converted into a semi-quantitative score?
• Why are controls and standardisation essential for reproducible IHC?
• How does protein-level detection complement transcript quantification?

Key concepts

• Antibody-antigen binding
• Antigen retrieval
• Chromogenic and fluorescent detection
• Semi-quantitative scoring (intensity and extent)
• Positive and negative controls
• Pre-analytical fixation effects

Mechanisms

A primary antibody binds its target antigen in a fixed tissue section; detection is amplified through secondary antibodies and enzyme- or fluorophore-linked systems that produce a coloured or fluorescent deposit at the antigen's location. Fixation and antigen-retrieval steps influence whether epitopes remain detectable, so pre-analytical handling strongly affects the result. Expression is read semi-quantitatively by judging staining intensity and the proportion of positive cells, often combined into a score; because this reading is observer- and protocol-dependent, validated controls and standardised procedures are needed for reproducibility. Professional guidelines illustrate how scoring and pre-analytical variables are standardised for receptor testing (Hammond et al., 2010; Wolff et al., 2013).

Clinical relevance

IHC is central to tissue diagnosis and to reporting protein biomarkers, and understanding its semi-quantitative nature is part of interpreting pathology reports. This entry describes the method and its standardisation; the cited guidelines are referenced to illustrate methodology and reporting, not to provide diagnostic cut-offs or treatment decisions for individual patients.

Evidence & guidelines

Standardisation of semi-quantitative IHC is exemplified by joint ASCO/CAP guidelines for estrogen and progesterone receptor testing (Hammond et al., 2010) and for HER2 testing in breast cancer (Wolff et al., 2013), which define pre-analytical handling, scoring, and control requirements. These are cited here as methodological references.

History

Antibody-based tissue staining developed from immunofluorescence in the mid-twentieth century, followed by enzyme-linked immunoperoxidase methods that allowed permanent, light-microscopy-visible staining. As protein biomarkers became central to pathology, attention shifted to standardising fixation, antigen retrieval, and scoring, formalised in consensus guidelines for receptor testing (Hammond et al., 2010; Wolff et al., 2013).

Debates

How reproducible is semi-quantitative IHC scoring?

Because staining intensity and extent are judged visually and depend on fixation and protocol, results can vary between observers and laboratories; standardised pre-analytics, validated controls, and defined scoring criteria are used to improve reproducibility.

Key figures

• M. Elizabeth Hammond
• Antonio Wolff
• D. Craig Allred

Seminal works

• hammond-2010
• wolff-2013

Frequently asked questions

Is immunohistochemistry quantitative?

IHC is best described as semi-quantitative: it estimates protein expression by judging staining intensity and the proportion of positive cells rather than producing an exact concentration, which is why standardised scoring and controls are important.

Why does tissue fixation affect IHC results?

Fixation and the subsequent antigen-retrieval step can mask or expose the epitopes that antibodies bind, so variation in pre-analytical handling can change staining and must be standardised for reliable results.