An immunological assay, also known as an immunoassay, is a laboratory technique used to detect or measure the presence or concentration of a specific substance, often called an analyte, in a solution. These tests leverage the highly specific reactions that occur within the immune system to identify various molecules. Immunoassays are employed across a wide range of fields for analyzing biological samples.
The Core Principle
Immunological assays are based on the precise interaction between antigens and antibodies. An antigen is a foreign substance that triggers an immune response, leading to antibody production. Antibodies are specialized proteins produced by the immune system that specifically bind to unique regions on an antigen, called epitopes. This binding is often compared to a “lock and key” mechanism due to its specificity.
The specific antigen-antibody binding creates an antigen-antibody complex, which forms the foundation for detection in immunological assays. This complex may not be directly visible or measurable, so various labels are incorporated to generate a detectable signal.
Labels can include enzymes, fluorescent tags, or radioactive isotopes. For instance, an enzyme-linked antibody binds to its target antigen. If a specific substrate is added, the enzyme catalyzes a reaction, producing a measurable change, such as a color change, light emission, or a fluorescent signal. This allows for qualitative (presence or absence) or quantitative (concentration) detection of the target analyte.
Why Immunological Assays Are Used
Immunological assays are used across many domains due to their specificity and sensitivity in detecting target molecules. In medical diagnostics, they are used to identify and monitor conditions. This includes detecting infectious diseases like HIV or hepatitis, by identifying disease-specific antigens or antibodies produced in response to infection.
These assays also play a role in assessing hormone levels, for example, in pregnancy tests or in monitoring thyroid function. They can also be used to monitor therapeutic drug levels in a patient’s blood or to detect tumor markers, which can aid in cancer diagnosis and progression monitoring.
Beyond clinical applications, immunological assays are used in scientific research to study protein expression and analyze cell populations. They are also used in drug development to screen for new therapies or evaluate the efficacy and safety of novel drugs. They are also applied in environmental testing and food safety, for instance, to detect contaminants or allergens.
Major Types of Immunological Assays
Several types of immunological assays leverage antigen-antibody interactions for detection. The Enzyme-Linked Immunosorbent Assay (ELISA) is a versatile method for detecting and quantifying biomolecules like proteins, antigens, or antibodies in biological samples.
In a common “sandwich ELISA,” a capture antibody is coated onto a multi-well plate. This antibody then binds to the target antigen present in the sample, effectively “sandwiching” it. A second enzyme-linked detection antibody is added, which also binds to the antigen. The enzyme’s activity is then measured by adding a substrate that produces a detectable signal, often a color change, allowing for quantification of the antigen.
Western Blot separates and identifies specific proteins within complex mixtures from cells or tissues. Initially, proteins are separated by size using gel electrophoresis, then transferred onto a solid membrane. The membrane is then probed with a primary antibody specific to the target protein, followed by a secondary antibody labeled with an enzyme. The enzyme’s reaction with a substrate produces a visible signal, confirming the presence and approximate size of the target protein.
Flow cytometry is a laser-based technique that analyzes individual cells as they flow in a fluid stream. Cells are often labeled with fluorescently tagged antibodies that bind to specific cell components, such as surface proteins. As these labeled cells pass through a laser beam, they emit light at different wavelengths. Detectors capture both scattered light, which provides information on cell size and granularity, and the emitted fluorescence, which identifies specific cellular properties or markers. This allows for the detection, counting, and even sorting of different cell populations.