A Guide to Immunohistochemistry Antibodies

Immunohistochemistry (IHC) is an essential technique that combines histological and immunological methods to visualize the distribution and localization of specific target antigens within tissue samples. Widely used in research and clinical pathology, IHC provides critical insights into disease mechanisms, enabling the identification of biomarkers and the diagnosis of conditions like infections and cancer. Central to the success of this method is the use of antibodies, which bind specifically to their target antigens, allowing researchers to detect and analyze proteins of interest. The accuracy and reliability of IHC results depend heavily on selecting the right antibodies, as improper choice or validation can lead to ambiguous findings or false results. Factors such as antibody specificity, affinity, and compatibility with the tissue and experimental conditions must be carefully considered. In this guide, we discuss useful considerations to help in choosing the most appropriate antibodies for use in IHC experiments.

The role of antibodies in IHC

Antibodies are central to the detection and visualization of specific antigens in IHC. Primary antibodies, which can be monoclonal or polyclonal, are critical to the specificity and sensitivity of the assay. Monoclonal antibodies bind to a single epitope on the antigen and generally excel in specificity. Polyclonal antibodies, on the other hand, recognize multiple epitopes, offering greater sensitivity.

Detection of the antigen-antibody interaction is facilitated by antibody labels, which can be direct or indirect. In the direct method, the primary antibody is labeled, simplifying the process but often requiring higher antibody concentrations due to limited signal amplification. In contrast, the more commonly used indirect method employs a labeled secondary antibody that binds to the primary antibody. Multiple secondary antibodies can bind to a single primary, allowing for signal amplification.

Antibody label considerations for IHC

The antibody label is an important factor that can enable or limit applications of IHC. Enzyme labels, such as horseradish peroxidase (HRP) and alkaline phosphatase (AP), are widely used due to their reliability. These rely on chromogenic reactions with substrates like DAB and Fast Red. However, tissues with high endogenous peroxidase (e.g., spleen and kidney) or phosphatase activity (e.g., kidney, intestine, and liver) can metabolize these substrates and lead to high background staining. This challenge necessitates careful selection and optimization, such as with specialized blockers or enzyme inactivators.

Fluorescent labels, like fluorescent dye conjugates, are essential for multiplexed detection and co-localization studies of multiple target antigens. Considerations for fluorescent labels include the use of specialized fluorescent microscopes and careful selection. When using multiple primary antibodies, they must be raised in different host species or isotypes to prevent secondary antibody cross-reactivity non-specific binding.

Avidin-biotin-based labeling can also be effective methods of detection. However, they can suffer from high background staining from endogenous biotin binding if not properly blocked. Polymer-based methods offer an alternative by employing multiple cross-linked peroxidase labels and secondary antibodies attached to a dextran polymer backbone. These methods enhance chromogenic detection, making them particularly valuable for applications demanding greater sensitivity. In choosing antibodies, users should consider which label and detection method will work best for the specific IHC experiment.

Direct versus indirect IHC detection

Directly conjugated primary antibodies simplify immunohistochemistry detection by eliminating the need for secondary antibodies, streamlining staining protocols, and reducing sample handling. This approach is particularly advantageous for preserving delicate tissues, as it minimizes wash cycles and potential damage. By avoiding secondary antibody usage, direct detection reduces the risk of non-specific binding, resulting in lower background noise and cleaner results. However, this method is generally more effective for highly expressed antigens, as low-abundance proteins can be challenging to detect.

Polymer labels can enhance the signal of primary antibodies, improving detection sensitivity. Additionally, innovations such as hapten-conjugated primary antibodies enable enhanced visualization by using fluorescently labeled, high-affinity anti-hapten secondary antibodies, providing new possibilities for direct detection in IHC.

Indirect detection using labeled secondary antibodies, due to signal amplification, remains to be a reliable method of boosting sensitivity. This method is particularly useful for studying low-abundance proteins or subtle expression differences in tissue samples.

Choose antibodies validated for IHC

Antibody validation is a critical step in ensuring reliable results in immunohistochemistry. Because the target protein may present itself in different forms depending on the application, it is essential to confirm that the chosen antibody performs well in IHC. This can be done by reviewing the manufacturer’s datasheet and observing the IHC staining data generated under validated conditions.

In the best case scenario, the antibody will have been validated for your specific tissue type, species, and format. For instance, if you require an antibody for IHC on mouse tissue, you will want to see mouse-specific IHC data. The sample format should also be similar, as an antibody validated for use with frozen samples may not recognize its target in FFPE tissue due to differences in protein conformation.

It is also important to remember that antibodies are only one component of an immunoassay, and other factors such as sample preparation, blocking and washing buffers, incubation time and temperature, and the detection method all play significant roles. Referencing the manufacturer’s recommended IHC staining protocol can provide valuable guidance on optimal experimental conditions. Careful consideration of these factors ensures robust, reproducible IHC results tailored to your experimental needs.

References

Magaki S, Hojat SA, Wei B, So A, Yong WH. An Introduction to the Performance of Immunohistochemistry. Methods Mol Biol. 2019;1897:289-298. doi:10.1007/978-1-4939-8935-5_25

Mason, E. Tips for Choosing Antibodies. Biocompare. 2018 May 14 [cited 2024 Dec]. Available from: www.biocompare.com/Bench-Tips/349787-Tips-for-Choosing-Antibodies/

Mason, E. Avoiding Common Immunohistochemistry Mistakes. Biocompare. 2023 Apr 20 [cited 2024 Dec]. Available from: www.biocompare.com/Editorial-Articles/595864-Avoiding-Common-Immunohistochemistry-Mistakes/

Mason, E. The Importance of Antibody Validation. Biocompare. 2023 Sep 19 [cited 2024 Dec]. Available from: www.biocompare.com/Editorial-Articles/599564-The-Importance-of-Antibody-Validation/

Mason, E. Tips for Successful IHC Staining. Biocompare. 2021 May 11 [cited 2024 Dec]. Available from: www.biocompare.com/Editorial-Articles/574807-Tips-for-Successful-IHC-Staining/

Mason, E. Optimizing Your Immunohistochemistry Workflow. 2020 Jan 30 [cited 2024 Dec]. Available from: www.biocompare.com/Editorial-Articles/558961-Optimizing-Your-Immunohistochemistry-Workflow/