Immunohistochemistry (IHC) uses antibodies to detect specific protein antigens in tissue sections, providing information about protein expression, localization, and tissue architecture that no lysate-based assay can match. IHC is central to surgical pathology, cancer diagnosis, and research into protein expression patterns. But IHC is also notoriously finicky — antigen retrieval conditions, antibody dilutions, and detection systems all require careful optimization. This guide covers the complete IHC workflow for both formalin-fixed paraffin-embedded (FFPE) and frozen tissue sections.

FFPE vs. Frozen Sections

| Parameter | FFPE | Frozen (Cryosections) | |-----------|------|----------------------| | Tissue morphology | Excellent | Good (some ice crystal artifact) | | Antigen preservation | Variable (formalin crosslinks mask epitopes) | Excellent (minimal fixation) | | Storage | Room temperature, years | −80°C, months | | Section thickness | 3–5 µm | 5–10 µm | | Antigen retrieval needed | Usually yes | Usually no | | Best for | Archived clinical specimens, tissue banks | Antigens destroyed by formalin, lipids, cell surface markers |

Most research and clinical IHC is performed on FFPE tissue. The protocol below focuses on FFPE with notes for frozen sections where they differ.

Step 1: Tissue Sectioning and Mounting

Cut 3–5 µm sections from FFPE blocks using a microtome.
Float sections in a 40–45°C water bath to flatten.
Mount on positively charged slides (e.g., Superfrost Plus, Thermo Fisher) to prevent section detachment during antigen retrieval.
Dry slides at 60°C for 1 hour or overnight at 37°C. Incomplete drying causes section loss.

For frozen sections: Cut 5–10 µm sections on a cryostat at −20°C. Mount on charged slides and air-dry for 30 minutes at room temperature. Fix in cold acetone (−20°C) for 10 minutes, or 4% paraformaldehyde for 10–15 minutes at room temperature, depending on the target antigen. Skip deparaffinization and antigen retrieval steps.

Step 2: Deparaffinization and Rehydration (FFPE Only)

Paraffin must be completely removed or antibodies will not penetrate the tissue:

| Solution | Duration | |----------|----------| | Xylene (or xylene substitute) | 3 × 5 minutes | | 100% ethanol | 2 × 3 minutes | | 95% ethanol | 2 × 3 minutes | | 70% ethanol | 1 × 3 minutes | | Deionized water | 2 × 3 minutes |

Tip: Incomplete deparaffinization is visible as waxy, hydrophobic patches on the section that repel aqueous solutions. If you see this, repeat the xylene steps with fresh solvent.

Step 3: Antigen Retrieval

Formalin fixation creates methylene bridges (crosslinks) between proteins that mask antibody epitopes. Antigen retrieval reverses these crosslinks. This step is often the most critical for IHC success.

Heat-Induced Epitope Retrieval (HIER)

The most commonly used approach. Slides are immersed in retrieval buffer and heated:

Common retrieval buffers:

Citrate buffer (pH 6.0): 10 mM sodium citrate, 0.05% Tween-20, pH 6.0. Works for most antibodies. This is the default starting point.
Tris-EDTA buffer (pH 9.0): 10 mM Tris base, 1 mM EDTA, 0.05% Tween-20, pH 9.0. Often more effective for nuclear antigens and some antibodies that fail with citrate.

Heating methods:

Pressure cooker: 15 psi for 3 minutes (after reaching pressure). Most consistent and effective.
Microwave: 10 minutes at full power (bring to boil), then 10 minutes at 20% power.
Water bath: 95–98°C for 20–40 minutes. Gentler but slower.
Steamer: 20–30 minutes. Convenient for batch processing.

After heating, let slides cool in the retrieval buffer for 20–30 minutes. Do not quick-cool — thermal shock damages tissue morphology.

Enzymatic Antigen Retrieval

Use when HIER damages the target epitope or tissue:

Proteinase K: 20 µg/mL in TBS, room temperature, 10–20 minutes
Pepsin: 0.1% in 0.01 M HCl, 37°C, 10–30 minutes
Trypsin: 0.05% in TBS with 0.1% CaCl₂, 37°C, 10–30 minutes

Enzymatic retrieval works well for extracellular matrix and basement membrane proteins (collagen IV, laminin) that are resistant to HIER.

Step 4: Blocking

Endogenous Peroxidase Block (Required for HRP-Based Detection)

Incubate sections in 3% hydrogen peroxide (H₂O₂) in methanol or TBS for 10–15 minutes at room temperature. This quenches endogenous tissue peroxidase that would generate false-positive signal. Perform this step before or after antigen retrieval (both approaches work; before retrieval is more convenient).

Endogenous Biotin Block (If Using Biotin-Avidin Detection)

Tissues rich in endogenous biotin (liver, kidney, brain) require blocking with an avidin-biotin blocking kit (Vector Laboratories, Cat# SP-2001): avidin solution for 15 minutes, rinse, biotin solution for 15 minutes.

Non-Specific Protein Block

Incubate sections in blocking solution for 30–60 minutes at room temperature:

5–10% normal serum from the species in which the secondary antibody was raised (e.g., normal goat serum for goat anti-rabbit secondary)
Alternative: 1–5% BSA in TBS-T (TBS + 0.1% Tween-20)
Commercial blockers: Background Sniper (Biocare Medical) or protein-free block (Dako/Agilent)

Do not use serum from the same species as the primary antibody — this introduces competing IgG.

Step 5: Primary Antibody Incubation

Dilute the primary antibody in antibody diluent (1% BSA in TBS-T, or a commercial diluent like Dako Antibody Diluent, Cat# S3022).

Typical dilutions: Start with the manufacturer's recommended IHC dilution (commonly 1:50 to 1:500). Optimize by testing a dilution series on positive control tissue.

Incubation conditions:

Standard: 1 hour at room temperature (faster, convenient)
Sensitive: Overnight (12–18 hours) at 4°C in a humidified chamber (better for weak antigens)

Do not let sections dry out at any point — this causes irreversible background artifacts.

Step 6: Secondary Antibody and Detection

Wash slides 3 × 5 minutes in TBS-T (or PBS-T) between each incubation step.

Polymer-Based Detection (Recommended)

Modern polymer systems like EnVision+ (Dako/Agilent), ImmPRESS (Vector Laboratories), or MACH 4 (Biocare Medical) attach multiple enzyme molecules (HRP or AP) directly to the secondary antibody polymer. No biotin or avidin needed, eliminating endogenous biotin issues.

Apply polymer-HRP conjugate. Incubate 30 minutes at room temperature.
Wash 3 × 5 minutes in TBS-T.

ABC (Avidin-Biotin Complex) Method

The classic amplification method:

Apply biotinylated secondary antibody (e.g., Vector BA-1000 for goat anti-rabbit) at 1:200 for 30 minutes.
Wash.
Apply ABC reagent (Vector Vectastain Elite ABC Kit, prepare 30 minutes before use) for 30 minutes.
Wash.

Step 7: Chromogenic Detection

DAB (3,3'-Diaminobenzidine)

The most common IHC chromogen. Produces a brown precipitate at the site of HRP activity.

Prepare DAB working solution: 1 drop (~20 µL) DAB chromogen + 1 mL DAB buffer (e.g., ImmPACT DAB, Vector Laboratories, Cat# SK-4105).
Apply to sections and monitor color development under the microscope (1–5 minutes typically).
Stop the reaction by rinsing in tap water when specific staining is visible but background is still clean.

Critical: DAB development time must be consistent across all slides for semi-quantitative comparison.

Alternative Chromogens

AEC (3-amino-9-ethylcarbazole): Red precipitate. Alcohol-soluble — must use aqueous mounting medium.
Permanent Red / Fast Red: Red precipitate for AP systems. Good contrast with DAB for double staining.
Vector VIP: Purple precipitate. Useful when DAB would be confused with melanin or hemosiderin pigment.

Step 8: Counterstain and Mounting

Counterstain with Mayer's hematoxylin for 30–60 seconds (provides blue nuclear contrast).
Rinse in running tap water for 5 minutes ("bluing").
Dehydrate: 70% ethanol → 95% ethanol → 100% ethanol (2× each, 2 minutes per step) → xylene (2 × 3 minutes).
Mount with a permanent mounting medium (e.g., Permount, Fisher, or DPX).

For AEC or aqueous-incompatible chromogens, skip dehydration and mount with aqueous medium (Glycergel, Dako).

Essential Controls

| Control | Purpose | |---------|---------| | Positive control tissue | Known-positive tissue confirms the protocol works | | Negative control (no primary antibody) | Omit primary; apply diluent only. Reveals non-specific secondary binding | | Isotype control | Replace primary with same-species, same-isotype irrelevant antibody at the same concentration | | Absorption control | Pre-incubate primary with excess immunizing peptide. Staining should disappear if antibody is specific |

Never publish IHC data without at least positive and negative controls.

Troubleshooting IHC

No Staining

Antigen retrieval inadequate: Try the alternative pH (citrate pH 6.0 vs. Tris-EDTA pH 9.0). Extend heating time.
Antibody not suitable for IHC on FFPE tissue: Some antibodies only work on frozen sections or Western blot. Check the datasheet.
Primary antibody too dilute: Test a higher concentration.
Detection system issue: Test on known positive tissue with a validated antibody.
Section dried out: Repeat on fresh slides with careful attention to humidity.

High Background

Insufficient blocking: Extend blocking time. Try a different blocking serum or add 0.1–0.3% Triton X-100 to permeabilize.
Primary antibody too concentrated: Dilute further. Test a dilution series.
DAB overdeveloped: Shorten incubation time. Monitor under the microscope.
Endogenous peroxidase not blocked: Ensure H₂O₂ step was not omitted.
Endogenous biotin (if using ABC): Switch to a polymer detection system.

Non-Specific Staining (Staining in Wrong Cell Compartment)

Antibody cross-reactivity: Validate with a different antibody targeting the same protein. Use the absorption control.
Fc receptor binding: Add 5–10% normal serum matching the tissue species in the blocking step.

Edge Staining or Section Detachment

Poor adhesion: Use Superfrost Plus slides. Dry completely before staining.
Sections lifting during HIER: Reduce pressure cooker time. Pre-bake slides at 60°C for 1 hour.

How LabProtocol.co Can Help

IHC optimization requires balancing antigen retrieval method, antibody dilution, detection system, and chromogen — and every antibody-tissue combination behaves differently. LabProtocol.co generates IHC protocols customized to your specific antibody, tissue type, and detection system, with recommended antigen retrieval conditions and dilution starting points. Create your IHC protocol and reduce optimization time.

Key Points

HIER with citrate pH 6.0 is the default starting point for most antibodies on FFPE tissue.
Polymer-based detection systems (EnVision, ImmPRESS) have largely replaced ABC for routine IHC.
Never let sections dry out during the staining procedure.
Optimize primary antibody dilution on positive control tissue before running experimental samples.
Include positive, negative, and isotype controls with every experiment.