Immunohistochemistry is a powerful technique for visualizing antigen distribution in tissue sections. Its success is highly dependent on meticulous attention to detail at every step, from tissue preparation to final interpretation.
This is the most critical phase, as poor preparation can ruin the experiment before it even begins.
Tissue Collection and Fixation:
Rapid Fixation: Tissues must be placed in fixative (typically 10% Neutral Buffered Formalin) immediately after dissection to prevent degradation (autolysis) and preserve antigenicity.
Fixation Time: Under-fixation leaves antigens vulnerable to degradation, while over-fixation can mask epitopes (the sites antibodies bind to), making them unrecognizable. Follow optimal fixation times for your tissue type (usually 24-48 hours for formalin).
Tissue Processing and Embedding:
Ensure complete dehydration and clearing to allow proper paraffin infiltration. Incomplete processing can make sections crumble or tear.
Sectioning:
Use positively charged or adhesive slides (e.g., poly-L-lysine coated) to prevent tissue detachment during rigorous washing and incubation steps.
Section thickness should be consistent (typically 4-5 µm). Thick sections can trap reagents and cause non-specific background.
Float sections on a warm water bath (~40-45°C) that is clean and free of contaminants to avoid artifacts on the tissue.
This step is often essential for formalin-fixed, paraffin-embedded (FFPE) tissues to reverse the cross-links formed during fixation and "unmask" the antigens.
Choice of Method: Determine whether Heat-Induced Epitope Retrieval (HIER) (using a pressure cooker, microwave, or water bath with citrate/EDTA buffer) or Protease-Induced Epitope Retrieval (PIER) is optimal for your specific antibody. The protocol must be empirically determined.
Consistency: Timing, temperature, and pH of the retrieval buffer are critical. Any deviation can drastically affect staining intensity and specificity.
Cooling: After HIER, allow the slides to cool naturally to room temperature in the retrieval buffer. Rapid cooling can damage the tissue morphology.
Blocking:
Always block endogenous peroxidase activity (e.g., with 3% H₂O₂ in methanol) for HRP-based detection systems before applying the primary antibody.
Block non-specific protein-binding sites by incubating with a normal serum (from the same species as the secondary antibody) or a protein block. This minimizes background staining.
Antibody Application:
Optimize Titration: The most common cause of failure is using an incorrect antibody concentration. Always perform a dilution series for both primary and secondary antibodies to find the optimal signal-to-noise ratio.
Negative Controls: These are non-negotiable for interpreting results correctly. You must include:
No Primary Antibody Control: Replace the primary antibody with buffer or an isotype control. This identifies background from the secondary antibody or detection system.
Isotype Control: Use an irrelevant immunoglobulin of the same class and species as the primary antibody. This identifies non-specific Fc receptor binding.
Positive Control: Include a tissue known to express the target antigen. This validates that the entire staining procedure worked correctly.
Incubation and Washing:
Incubate slides in a humidified chamber to prevent evaporation of reagents, which would concentrate antibodies and increase background.
Wash thoroughly between steps with an appropriate buffer (e.g., PBS or TBS) to remove unbound antibody and minimize non-specific binding.
Detection System: Choose an appropriate detection system (e.g., Avidin-Biotin Complex (ABC) or Polymer-based systems). Polymer systems are often more sensitive and avoid endogenous biotin issues.
Chromogen Selection: DAB (brown) is the most common chromogen for HRP. It produces an insoluble precipitate.
Develop Under Microscopic Control: Monitor the development of the chromogen reaction under a microscope to prevent over-development, which leads to high background, or under-development, which gives a weak signal.
Stop the reaction by immersing slides in distilled water as soon as the desired intensity is reached.
Counterstaining: Use a light counterstain, such as Hematoxylin, to provide morphological context. Over-counterstaining can obscure the specific IHC signal.
Dehydration: After aqueous mounting media (for immunofluorescence) or before organic mounting media, dehydrate the sections through a graded series of alcohols and a clearing agent (e.g., xylene) to ensure clarity and permanence.
Mounting: Use a compatible mounting medium. For DAB stains, use a permanent, organic-based mounting medium. For fluorescent signals, use an anti-fade mounting medium to slow photobleaching.
Localization is Key: Correctly identify the cellular compartment of the signal (nuclear, cytoplasmic, membranous). An off-target signal is a sign of non-specificity.
Artifact Recognition: Learn to distinguish true positive staining from common artifacts like edge artifact (strong staining at tissue edges), drying artifact, or precipitated chromogen.
Hazardous Reagents: Many IHC reagents are hazardous (e.g., formamide, DAB, organic solvents, xylene).
Precaution: Always consult Safety Data Sheets (SDS). Handle reagents in a fume hood, and wear appropriate PPE: gloves, lab coat, and safety glasses.
Dispose of chemical waste according to institutional regulations.
By systematically addressing these precautions, you significantly increase the reliability, reproducibility, and interpretability of your IHC experiments.