Immunohistochemistry (IHC): A Technique Training Guide for Biomedical Scientists

14 min read · Published: 2025-01-08 · Updated: 2026-06-08

Immunohistochemistry (IHC) sits at the heart of modern cellular pathology, turning a routine tissue section into a map of protein expression that drives diagnosis, prognosis and treatment selection. For the biomedical scientist (BMS), IHC is both a technical craft and a regulated diagnostic service in which every step from fixation through antigen retrieval to detection and interpretation must be controlled, validated and quality-assured. This guide walks NHS trainees and registered staff through the full IHC workflow, the critical role of controls, the interpretation of predictive markers such as HER2, ER and PR, and a systematic approach to troubleshooting, framed by current UK NEQAS for Immunocytochemistry & In-Situ Hybridisation (UK NEQAS ICC & ISH), Royal College of Pathologists (RCPath) and ISO 15189:2022 expectations.

What IHC Is and Why It Matters

IHC uses antibodies to localise specific antigens (usually proteins) within tissue sections, visualised through an enzyme-substrate reaction that produces a coloured deposit, most commonly brown using 3,3'-diaminobenzidine (DAB) as the chromogen. Because the reaction is anchored to tissue architecture, the pathologist can see not just whether a protein is present but exactly where, in which cell type and in which subcellular compartment (nucleus, cytoplasm or membrane).

In NHS cellular pathology, IHC supports several distinct purposes:

Diagnostic markers that classify tumours (for example, cytokeratins for epithelial origin, CD45 for lymphoid lineage, S100 and SOX10 for melanocytic lesions).
Prognostic markers that predict likely behaviour (for example, Ki-67 proliferation index).
Predictive markers that guide targeted therapy and must meet the highest analytical standards (for example, HER2, oestrogen receptor (ER) and progesterone receptor (PR) in breast cancer, and PD-L1 in several tumour types).

Because predictive IHC directly determines whether a patient receives a specific drug, these assays are treated as high-risk under ISO 15189:2022 and are subject to particularly rigorous validation, internal quality control (IQC) and external quality assessment (EQA).

The IHC Workflow Step by Step

A reproducible result depends on every pre-analytical and analytical variable being controlled. The classic indirect, polymer-based detection workflow runs as follows.

1. Fixation. Tissue is fixed in 10% neutral buffered formalin (NBF). Both under-fixation and over-fixation distort antigenicity, so cold ischaemic time and total fixation time are critical pre-analytical variables. For breast predictive markers, recognised guidance specifies a fixation window (broadly 6 to 72 hours in NBF) precisely because ER, PR and HER2 are sensitive to fixation. 2. Processing and embedding. Dehydration, clearing and paraffin wax embedding produce a formalin-fixed paraffin-embedded (FFPE) block. 3. Sectioning. Sections are cut at around 3-4 micrometres and mounted on charged (adhesive-coated) slides to withstand antigen retrieval. 4. Dewaxing and rehydration. Wax is removed (xylene or a substitute) and sections are taken through graded alcohols to water. On automated platforms this is integrated into the run. 5. Antigen (epitope) retrieval. Formalin fixation creates protein cross-links that mask epitopes; retrieval reverses this (see next section). 6. Blocking. Endogenous peroxidase is quenched (commonly with hydrogen peroxide) to prevent false-positive background with peroxidase-based detection. Protein blocking may also be used to reduce non-specific binding. 7. Primary antibody. The validated primary antibody is applied at an optimised dilution and incubation time/temperature to bind its target epitope. 8. Detection. A labelled polymer detection system (secondary antibody conjugated to enzyme-bearing polymer) amplifies the signal. Modern systems are biotin-free, avoiding endogenous biotin artefact. 9. Chromogen development. DAB produces an insoluble brown precipitate at the antigen site. 10. Counterstaining and mounting. Haematoxylin provides nuclear contrast; sections are dehydrated, cleared and coverslipped.

The overwhelming majority of NHS IHC is now performed on automated immunostainers, which improve reproducibility and traceability. Even so, the BMS remains responsible for protocol validation, control performance and recognising when a run has failed.

Antigen Retrieval Methods

Antigen retrieval is the single most influential analytical step and a leading cause of insufficient staining when it goes wrong. There are two broad approaches.

Heat-induced epitope retrieval (HIER) is the most widely used method. Sections are heated in a buffered solution to break formalin-induced cross-links and unmask epitopes. Heat may be delivered by pressure cooker, microwave, water bath, steamer or, most commonly today, the on-board retrieval module of an automated platform. The retrieval buffer pH is critical:

Low-pH buffers (around pH 6), typically citrate-based, suit some epitopes.
High-pH buffers (around pH 8-9), typically Tris-EDTA or EDTA-based, are optimal for many nuclear and membranous antigens and are frequently the better choice for predictive markers.

Proteolytic (enzymatic) induced epitope retrieval (PIER) uses proteases such as trypsin, pepsin or proteinase K to digest cross-links. It is harder to standardise, as over-digestion destroys morphology and under-digestion fails to unmask the epitope. Some antibodies require enzyme retrieval, others heat, and a few a combination.

The correct retrieval method, buffer, pH, temperature and time are antibody-specific and must be established during validation, not assumed. The UK NEQAS ICC & ISH Best Methods database, which collects anonymised protocols from participants achieving excellent staining, is a valuable starting point, but any protocol drawn from it must be validated in-house before clinical use.

| Variable | Heat-induced (HIER) | Enzymatic (PIER) | |----------|---------------------|------------------| | Mechanism | Heat breaks formalin cross-links | Protease digests cross-links | | Typical buffers | Citrate (~pH 6), Tris-EDTA (~pH 8-9) | Trypsin, proteinase K, pepsin | | Main control variables | pH, temperature, time | Enzyme concentration, time, temperature | | Common failure | Under-retrieval (weak/false-negative) | Over-digestion (loss of morphology) | | Best for | Most nuclear and membrane antigens | A subset of antigens needing digestion |

Controls: The Foundation of Reliable IHC

No IHC slide can be interpreted safely without appropriate controls, and their use is an explicit expectation of UK NEQAS ICC & ISH and ISO 15189:2022. Controls confirm that the assay performed as intended on that run and that staining reflects true antigen expression rather than technical artefact.

Positive control tissue is known to express the target antigen and ideally contains a range of expression levels (high and low). A correctly stained positive control confirms the assay detected the antigen with appropriate sensitivity. For low-expressing thresholds (such as HER2 1+), a control spanning weak to strong expression is especially valuable.
Negative control tissue is known not to express the antigen and confirms the assay is not producing non-specific or false-positive staining.
Internal (built-in) controls are normal cell populations within the patient section that are expected to express, or not express, the target (for example, normal breast epithelium acting as an internal benchmark for ER). Internal controls are powerful because they experience the identical pre-analytical and analytical conditions as the tumour.
No-primary-antibody (reagent) control omits the primary antibody to confirm that any signal arises from genuine antigen detection and not from the detection system, endogenous enzyme or tissue itself.

Many laboratories mount the patient section and a control on the same slide (on-slide control) so both share the run conditions exactly. Control performance must be reviewed and documented for every run before any clinical case is authorised; a failed control invalidates the run.

Predictive Markers: HER2, ER and PR

Breast predictive markers are the most scrutinised IHC assays in the NHS because they directly determine eligibility for hormonal and HER2-targeted therapy. Reporting follows internationally recognised criteria (notably the ASCO/CAP recommendations) within an RCPath dataset framework, and UK laboratories participate in dedicated EQA modules.

HER2 (human epidermal growth factor receptor 2) is scored by the intensity and completeness of membrane staining in invasive tumour cells:

| Score | Membrane staining pattern | Status | |-------|---------------------------|--------| | 0 | No staining, or faint/barely perceptible incomplete staining in ≤10% of tumour cells | Negative | | 1+ | Faint/barely perceptible incomplete membrane staining in >10% of tumour cells | Negative | | 2+ | Weak-to-moderate complete membrane staining in >10% of cells, or strong complete staining in ≤10% | Equivocal | | 3+ | Strong, complete, circumferential membrane staining in >10% of tumour cells | Positive |

A 3+ result should be readily discernible at low magnification within a homogeneous, contiguous invasive population. An equivocal 2+ result must be reflexed to in-situ hybridisation (ISH) to assess gene amplification. The clinical introduction of antibody-drug conjugates effective in tumours with low HER2 protein has made accurate discrimination of IHC 0 versus 1+ clinically important; both remain "negative" by traditional ASCO/CAP criteria, but reliable distinction now influences treatment access. Best practice for borderline cases includes reviewing at high power (40x), using controls that span the expression range, and seeking a second observer for cases near a threshold.

ER and PR are nuclear hormone receptors. They are reported as the percentage of tumour cell nuclei staining and the staining intensity, commonly combined into a semi-quantitative score such as the Allred (Quick) score, which sums a proportion score (0-5) and an intensity score (0-3) for a total of 0-8. In current UK practice a tumour is regarded as hormone-receptor positive when at least 1% of invasive tumour cell nuclei show definite staining; below this threshold the case is reported negative, though very low-level expression should be commented upon. Robust internal controls (normal duct epithelium) are essential, because a falsely negative ER result could wrongly deny a patient effective endocrine therapy.

Accurate predictive scoring depends on excellent technique: correct fixation, validated retrieval and detection, and well-characterised controls. A single analytical drift can shift a case across a clinically decisive threshold.

Validation and Quality Assurance under ISO 15189:2022

Under ISO 15189:2022, the standard against which UKAS (United Kingdom Accreditation Service) assesses UK medical laboratories, every IHC assay must be fit for purpose before clinical use. The standard distinguishes:

Verification — confirming that an assay performs to the manufacturer's stated specification when used as intended (appropriate for in-vitro diagnostic CE/UKCA-marked kits used per instructions).
Validation — establishing performance for assays used outside their stated intended use, modified, or laboratory-developed (often termed laboratory-developed tests). Many IHC protocols, where antibody, retrieval or platform parameters are optimised in-house, fall here and require fuller validation.

Practical IHC validation and ongoing quality assurance include:

Establishing optimal antibody dilution, retrieval and incubation conditions, and assessing sensitivity and specificity against known positive and negative tissues.
Confirming reproducibility across runs, operators and (where relevant) platforms.
Documenting acceptance criteria, control requirements and authorisation rules in standard operating procedures (SOPs).
Running IQC (controls) with every batch and recording performance, with lot-to-lot verification when antibody or detection reagent batches change.
Participating in EQA through UK NEQAS ICC & ISH, with documented review and corrective action for any underperformance.
Risk assessment, a strengthened theme in ISO 15189:2022, focusing on the clinical impact of an erroneous result, which is why predictive markers attract the most rigorous controls.

EQA is not a tick-box exercise: schemes such as UK NEQAS ICC & ISH have improved the accuracy and inter-laboratory consistency of predictive marker testing across the UK.

Troubleshooting Common IHC Problems

Most insufficient IHC results are caused by a small number of recurring issues. Experience from external quality schemes consistently shows that suboptimal or incorrect epitope retrieval and poorly calibrated antibody dilutions are leading causes, and that the great majority of failures present as weak or false-negative staining rather than excessive background. A systematic approach helps.

No staining or weak staining (often false-negative):

Check antigen retrieval method, buffer pH, temperature and time first; this is the most common culprit.
Confirm antibody concentration, incubation time and reagent expiry; review whether a new lot was verified.
Verify the positive control stained correctly; if it failed, the run is invalid regardless of the patient result.
Consider pre-analytical causes: cold ischaemic time, delayed or inadequate fixation, or over-decalcification of bone.

Excessive or non-specific background:

Inadequate endogenous peroxidase blocking, or endogenous biotin with older biotin-based systems.
Antibody too concentrated or incubated too long; insufficient washing between steps.
Section drying out during the run, edge artefact, or tissue folds and overlapping cells.

Patchy, uneven or unexpected patterns:

Poor section adhesion or thick/uneven sections.
Necrotic, crushed or poorly fixed tissue giving artefactual staining.
Wrong subcellular localisation (for example, cytoplasmic staining where a nuclear pattern is expected) suggests a specificity or interpretation issue and warrants review.

The cardinal rule is to interpret the controls before the case: a failed positive or negative control means the run cannot be reported, and the cause must be investigated and corrected. Recurrent failures should be managed through the laboratory's quality system using root cause analysis and corrective and preventive action (CAPA).

Frequently Asked Questions

Why is antigen retrieval necessary in IHC?

Formalin fixation cross-links proteins, which masks the epitopes that antibodies need to recognise. Antigen retrieval, usually heat-induced (HIER) in a buffered solution or, less often, enzymatic, reverses this masking so the antibody can bind. Without correct retrieval, many antigens give weak or false-negative results, which is why it is the most influential analytical step in the workflow.

What controls must be run with every IHC assay?

Each assay needs a positive control tissue known to express the antigen (ideally across a range of intensities), a negative control known not to express it, and where available an internal control within the patient section. A no-primary-antibody control confirms that signal arises from genuine antigen detection rather than the detection system. Control performance must be reviewed and documented before any case is authorised.

How is HER2 scored on immunohistochemistry?

HER2 is scored by the completeness and intensity of membrane staining in invasive tumour cells: 0 and 1+ are negative, 2+ is equivocal and reflexed to in-situ hybridisation, and 3+ (strong, complete, circumferential staining in more than 10% of cells) is positive. Reliable distinction of 0 versus 1+ has become clinically important because newer therapies can benefit tumours with low HER2 expression. Scoring follows internationally recognised ASCO/CAP criteria within RCPath reporting datasets.

What is the threshold for a positive ER or PR result?

In current UK practice, a breast tumour is reported as hormone-receptor positive when at least 1% of invasive tumour cell nuclei show definite staining; below this it is negative, though very low expression should be commented on. Results are commonly expressed semi-quantitatively using the Allred (Quick) score, combining proportion and intensity. Strong internal controls (normal duct epithelium) are essential to avoid a falsely negative result denying a patient endocrine therapy.

What is the difference between validation and verification of an IHC assay?

Under ISO 15189:2022, verification confirms that an assay performs to the manufacturer's stated specification when used exactly as intended, whereas validation establishes performance for assays that are laboratory-developed, modified, or used outside their stated intended use. Many optimised IHC protocols require validation, including assessment of sensitivity, specificity, reproducibility and appropriate controls, all documented in the SOP.

Most of my IHC failures are weak staining; where should I start?

Begin with antigen retrieval, since incorrect or insufficient retrieval is one of the most common causes of weak and false-negative staining, then check antibody dilution, incubation conditions and reagent expiry. Confirm whether the positive control also failed, which would invalidate the whole run, and review pre-analytical factors such as fixation time and cold ischaemic time. Persistent problems should be investigated formally through your quality system.

Further training

Build your cellular pathology and quality skills with related guides on NHS Laboratory Training and these companion articles:

Microtomy and Tissue Processing in Histology — the pre-analytical foundation for good IHC sections.
Method Validation and Verification under ISO 15189 — applying validation principles to laboratory assays.
UKAS and ISO 15189 Accreditation Guide — how accreditation underpins diagnostic quality.
Root Cause Analysis and CAPA in Pathology — managing assay failures through the quality system.