Different diagnostic tests require other immunological processes to detect antibodies and antigens. Immunofluorescence assay is one of the most commonly used immunological tests.
Immunofluorescence is the combination of the two words immuno and fluorescence. Immuno means immune or immunity; fluorescence implies fluorescent molecules that produce visible or invisible radiation.
Immunofluorescence assay is a significant technique commonly used in immunology or molecular biology for detecting the presence and distribution of specific proteins or antigens. This process uses antibodies labeled with fluorescent molecules that bind to the target protein or antigen of interest.
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Principle of Immunofluorescence Assay
Immunofluorescence assay is based on specific antibodies for detecting and visualizing particular proteins or antigens in biological samples using fluorescence microscopy.
The immunofluorescence assay principle relies on the antibodies’ specificity for their target proteins. This specificity helps selectively label and visualize the location and distribution of specific proteins within cells, tissues, or other biological samples.
Immunofluorescence can be helpful in clinical diagnostics for detecting specific antigens or markers associated with diseases or conditions.
The essential components of the immunofluorescence technique are antibodies and fluorescent labels.
- Antibodies: The antibodies are proteins the immune system produces that specifically bind to antigens. Likewise, the immune system recognizes antigens as foreign or non-self. In the immunofluorescence assay, specific antibodies target and attach to the protein or antigen of interest within a biological sample.
- Fluorescent Labels: Fluorophores, or fluorescent labels, are attached to the antibodies. These labels emit fluorescent light when exposed to specific wavelengths of light. Each fluorophore emits light at a unique wavelength, which helps identify and differentiate different targets in the sample.
General Steps of Immunofluorescence Assay
A general overview of the steps involved in an immunofluorescence experiment includes sample preparation, fixation, permeabilization, blocking, primary antibody incubation, washing, secondary antibody incubation, washing, mounting, and imaging.
- Sample Preparation: In the case of cells, culture them on glass coverslips or chamber slides. While using tissues, prepare tissue sections by fixing, embedding, and sectioning the tissue.
- Fixation: Chemical fixative (like formaldehyde) helps preserve the structure and immobilize the proteins while fixing the cells or tissues.
- Permeabilization: Permeabilization is often necessary to enter antibodies inside the cells or tissues. This step involves treating the sample with the help of detergents or other agents.
- Blocking: This step help to reduce the nonspecific binding of antibodies. Here, incubate the sample using a blocking solution like bovine serum albumin or normal serum.
- Primary Antibody Incubation: Apply the primary antibody, specific to the target protein, to the sample. The antibody binds to the target protein within the sample.
- Washing: The use of buffer solution helps in removing excess primary antibodies.
- Secondary Antibody Incubation: Use an antibody conjugated to a fluorescent dye in this step. This secondary antibody recognizes the primary antibody and binds to it.
- Washing: Like step 6, buffer solution helps remove excess secondary antibodies.
- Mounting: Use a mounting medium with anti-fading agents for mounting the sample to preserve the fluorescence signal.
- Imaging: The fluorescence microscope with appropriate filters examines the sample. This helps visualize the fluorescent signal emitted by the secondary antibody bound to the target protein. Different fluorophores can emit different colors of light, allowing for multicolor imaging.
Types of Immunofluorescence Assay
There are different types of immunofluorescence assays with slight variations in the use and types of antibodies used. Common types of immunofluorescence assay are flow cytometry, immunohistochemistry, direct, indirect, multiplex, and double immunofluorescence assay.
- Direct Immunofluorescence or direct fluorescent antibody (DFA) test is a simple method requiring only primary antibody. The primary antibody conjugates to a fluorophore (fluorescent dye), which directly binds to the antigen of interest in the sample.
- Indirect Immunofluorescence or indirect fluorescent antibody (IFA) test is the most common immunofluorescence technique. Firstly, application of a primary antibody specific to the targeted antigen occurs. Then, there is the use of a secondary antibody labeled with a fluorescent dye particular to the primary antibody. Here, the secondary antibody binds to primary antibodies. The added step makes this process more sensitive.
- Immunohistochemistry (IHC) combines immunofluorescence with tissue sectioning. Here, treatment of tissue sections with antibodies labeled with fluorescent dyes occurs. It helps in visualizing the distribution and localization of specific antigens within tissues.
- Flow Cytometry combines immunofluorescence with analysis of individual cells in a liquid stream. Each cell is labeled with fluorescently tagged antibodies and passed through a flow cytometer. The flow cytometer measures the fluorescence intensity of each cell. It is applicable in cell sorting, cell phenotyping, and quantifying surface markers on cells.
- Immunocytochemistry helps to detect specific proteins within cultured cells. Cells are fixed and then treated with labeled antibodies for visualization.
- Western Blotting is applicable in detecting and quantifying proteins. Here, gel electrophoresis combines with immunofluorescence, where protein separation occurs by size in the gel. Researchers then transfers protein to a membrane and probed with antibodies labeled with fluorescent dyes.
- Multiplex immunofluorescence helps detect multiple antigens simultaneously within a single sample. Here, different primary antibodies are labeled with fluorophores, which helps visualize multiple antigens or markers in a single experiment.
- Double immunofluorescence detects two different antigens within the same sample. Here, there is use of two primary antibodies from other species. Recognition of each occurs by a different secondary antibody labeled with distinct fluorophores. This method helps in studying co-localization or interactions between two proteins.
Immunofluorescence assay is useful widely in biomedical research and diagnostics because of several key benefits. The benefits are briefly explained below:
- Immunofluorescence is highly specific in detecting target proteins due to antibodies binding specifically to the interested antigen.
- This method is susceptible because fluorescent dyes amplify the signal, helping detect protein even in low-abundance samples.
- It helps visualize the distribution and location of proteins within cells and tissues. It also provides insights into the spatial organization of cellular structures, including subcellular compartments, cellular membranes, and organelles.
- Immunofluorescence helps in studying protein-protein interactions and co-localization within the same cellular compartment.
- A type of immunofluorescence helps in detecting multiple proteins and antigens within a single sample.
- This technique applies to various biological samples, including cells, tissue, and whole organisms. It is also helpful in cell biology, neuroscience, immunology, pathology, and diagnostics.
- This method also helps quantify proteins or antigens with appropriate imaging and analysis tools.
- Using fluorescence microscopy helps in obtaining detailed images of cellular structures and proteins.
- Likewise, this technique is compatible with other techniques like electron microscopy for visualizing the ultrastructure of cells and tissues.
- The signals from fluorescence are stable. Also, the signals can be preserved for imaging and analysis over time, which helps reanalyze the sample.
Although immunofluorescence is highly beneficial, it has some limitations. Researchers must consider these limitations and challenges before selecting this method. Some of the critical limitations of immunofluorescence assays are:
- Sometimes, false positive results may occur due to non-specific binding of antibodies to unrelated molecules. It may also yield false negative results due to low antigen expression or epitope masking.
- Cross-reactions with closely related antigens may occur. So, antibody validation and specificity testing are essential to address this issue.
- Selecting appropriate and quality antibodies is a challenging and vital step in immunofluorescence assay.
- The background signal from the sample can interfere with the specific signal, so proper controls and background subtraction methods are necessary to address the issue.
- While performing tissue-based immunofluorescence, fixation, and permeabilization methods can impact the results because fixation methods can alter protein structure, affecting antibody binding.
- Fluorescent dyes can be sensitive to photobleaching, reducing the intensity of the fluorescence signal over time. Hence, researchers must minimize exposure to intense light.
- Accurate quantification of immunofluorescence signals can be challenging due to variations in fluorescence intensity, background noise, and the dynamic range of detection.
- This method can be expensive and time-consuming. Also, this method requires expertise.
- Im, K., Mareninov, S., Diaz, M. F. P., & Yong, W. H. (2019). An Introduction to Performing Immunofluorescence Staining. Methods in molecular biology (Clifton, N.J.), 1897, 299–311. https://doi.org/10.1007/978-1-4939-8935-5_26
- The Principle of Immunofluorescence Assays. Retrieved from https://ibidi.com/content/364-the-principle-of-immunofluorescence-assays