Enzyme-linked immunosorbent assay, or ELISA, is a plate-based technique many labs employ to identify and quantify specific analytes, such as antigens or antibodies, within a liquid sample. In this process, antibodies, protein molecules created by the immune system to combat harmful foreign antigens, are immobilized on a polystyrene plate (typically in 96 or 384-well formats). By measuring the interaction between these molecules, researchers can gain critical insights into a wide range of biological samples.
ELISA’s primary steps are as follows:
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Antibody or protein binding and immobilization
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Coating every bound well with blocking material
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Several washes to eliminate any nonspecific binding
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Substrate application to generate a colorimetric signal
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Examining the plate
How Does ELISA Test Work?
The principle of ELISA dictates that if you are not using a kit with a plate that has already been pre-coated with antibodies, the first step is to coat a 96-well polystyrene plate with a target antibody or antigen.
The next step is blocking. The principle of blocking in an ELISA is that every unbound site on the plate is covered with a blocking compound. After several washes, the plate is then subjected to incubation with an enzyme-conjugated antibody. An additional round of washes eliminates any unbound antibodies. The next step is to add a substrate, which produces a colorimetric signal. A plate reader then concludes the process.
Each ELISA phase involves multiple wash steps to eliminate unbound material because the test relies on surface binding for specificity. Any surplus liquid must be thoroughly drained during this procedure to ensure that the solutions introduced in the subsequent stages remain undiluted. The use of specialized plate washers helps to ensure consistency and accuracy.
Particularly when determining the protein content in diverse samples like blood, ELISAs can be complex and include numerous intervening steps. The most intricate and variable stage of the process is detection, where several levels of antibodies can help amplify the signal.
Types of ELISAs
The common ELISA types available include:
Direct ELISA Principle
The application of a Direct ELISA involves using an antibody directly coupled to an enzyme to identify an antigen coated onto a multi-well plate. If there are no ELISA kits available for purchase to detect your target protein, consider using this detection approach.
Indirect ELISA
The antigen coated to a multi-well plate is detected in two stages or layers. First, a primary antibody that is specific to the antigen but is unlabeled is applied. This antibody is then bound to an enzyme-labeled secondary antibody.
The secondary antibody is typically an anti-species polyclonal antibody. The most widely used ELISA format, the indirect assay, has the following benefits and drawbacks:
Pros
The advantages of an indirect ELISA test include:
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There are many different labeled secondary antibodies commercially available.
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It is highly adaptable because several primary antibodies from a given species can be detected using a single labeled secondary antibody.
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Since the primary antibody is unlabeled, its immunoreactivity is fully preserved.
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Each primary antibody has many epitopes to which the labeled secondary antibody can bind, amplifying the signal and increasing sensitivity.
Cons
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There is a possibility of cross-reactivity with the secondary antibody, which may result in a nonspecific signal.
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The process requires an additional incubation step.
Sandwich ELISA
Sandwich ELISA applications frequently require the use of matched antibody pairs, each specific to a distinct, non-overlapping portion (epitope) of the antigen complex. The wells are coated with a first antibody, which is referred to as the “capture antibody.”
The well is subsequently filled with the sample solution. This phase is followed by adding a second antibody, referred to as the “detection antibody,” to assess the concentration of the analyte.
Competitive ELISA
In a competitive ELISA (also known as an inhibitory ELISA), the primary antibody competes for binding sites between antigens in the sample and those already coated on the wells of a microtiter plate.
Applications of ELISA Kits
The various types of ELISA, such as the double-antibody sandwich ELISA, indirect ELISA, and competitive ELISA, have led to a wide range of applications in fields as diverse as medicine, forensic science, and food science.
1. Diagnostic Procedures
In medical diagnosis, ELISA has established itself as a reliable platform used worldwide to detect a wide range of diseases in both humans and animals. There are various ELISA kits available today for detecting HIV, influenza, Dengue fever, Ebola, Chagas disease, leishmaniasis, Lyme disease, and West Nile virus.
The ELISA approach is also receiving growing interest in plant pathology (phytopathology). ELISA has effectively addressed the shortcomings of traditional serological analyses used in plant disease diagnosis.
In addition, facilities use these biomarkers for the detection of cancer, infectious diseases, and autoimmune disorders. ELISA is also vital for endocrinology diagnostic tests, monitoring cytokines, cardiac markers, insulin resistance, nephrology, thyroid activity, reproductive health, growth markers, skeletal and mineral metabolism, apoptosis, matrix metalloproteinases (MMPs), biogenic amines, and food safety analytics.
2. Humoral Immunity
ELISA has also shown considerable promise in examining the immune system's humoral reactivity to several infections. Antibodies and other constituents in bodily fluids play a role in the humoral immune response; consequently, the monitoring and quantification of these constituents are of the utmost significance.
3. Immunology
The immune system acts as the body's protector and functions through both innate and adaptive mechanisms (including cellular and humoral responses). The ability to measure and track immune response fluctuations is a cornerstone for comprehending immunological illnesses. ELISA has been proven repeatedly to be the most reliable and cost-effective approach for performing these kinds of measurements and monitoring across a wide range of scientific contexts.
4. Testing for Pregnancy to Cancer Detection
Cancer biomarkers are among the biomolecular entities that present the most significant challenge when used as target analytes. Fortunately, ELISA techniques have made significant strides in recent years, improving the potential for identifying these biomarkers.
Discovering reliable cancer biomarkers for early diagnosis is an ongoing endeavor. The clinical application of ELISA has enabled the detection of tumors at earlier stages, particularly in ovarian and breast cancers.
In addition, ELISA tests can assess the presence of controlled substances such as cannabinoids, amphetamines, opiates, cocaine, benzodiazepines, and methadone in urine samples. The technique can also track drug levels in individuals receiving treatment, such as monitoring anti-drug antibodies in patients with rheumatoid arthritis or inflammatory bowel disease.
Pregnancy triggers the expression of several distinct biomolecular substances, such as human chorionic gonadotropin (hCG), luteinizing hormone (LH), follicle-stimulating hormone (FSH), estriol (E3), and thyroid-stimulating hormone (TSH). Several of these proteins are detectable during the first trimester via ELISA using a mother's blood, saliva, or urine.
The most specific hormone ELISA identifies within the first month following fertilization is hCG. By the sixth week of pregnancy, an ELISA test can also identify the presence of estriol (E3) in a pregnant woman's saliva. Due to the development of commercial ELISA kits, these tests are now accessible at home, providing significant convenience.
5. Testing And Virus Detection
The advantages of employing ELISA to identify viruses include its applicability in developing nations, where infection rates are frequently high. ELISA's portability allows healthcare providers to reach the most vulnerable populations with on-site screening and immediate results, enabling opportunistic screening (e.g., within emergency departments).
HIV (Human Immunodeficiency Virus)
The first widely used HIV testing kit was an ELISA kit. Modern fourth-generation ELISA tests are the gold standard for screening, as they can simultaneously detect HIV-1/2 antibodies and the p24 antigen. This significantly reduces the "window period" compared to older tests, allowing for earlier diagnosis and treatment.
West Nile Virus
The West Nile virus is diagnosed using an IgM antibody-capture (MAC) ELISA of a subject's serum or cerebrospinal fluid (CSF) sampled 8 to 21 days after the onset of the disease. This specific method is highly effective at determining whether the virus has spread to the patient's central nervous system.
Newcastle Disease Virus (NDV)
NDV is a vesicular disease in birds that can be passed to humans. Depending on the strain, NDV can range in severity from mild respiratory problems to life-threatening clinical signs. ELISA is used to track the prevalence of NDV in a community, assisting in vaccination campaign planning and identifying infected flocks, ranging from the deadliest (velogenic) to less severe (mesogenic and lentogenic) variants.
6. Dietary Allergens
Allergenic proteins can be detected using ELISA assays by binding specific antibodies to the target allergen and observing the resulting colorimetric change. In the United States and many other regions, manufacturers are legally required to label their products for major allergens. Commonly recognized allergens include peanuts, tree nuts, soybeans, eggs, wheat, milk, fish, and shellfish. It is estimated that these eight allergens are linked to over 90% of all recorded food allergy cases.
Testing protocols in the food industry have historically centered on identifying pathogens that could cause illness or outbreaks, specifically fungal and bacterial infections. However, as technology has enabled high-throughput automated processing, contaminants like food allergens have become a critical focus for food safety.
Consequently, a significant number of manufacturers rely on ELISA applications within the food industry. It is the primary tool for detecting allergens such as those found in milk, peanuts, walnuts, almonds, and eggs. ELISA can also be used to verify food authenticity, a method highly beneficial in preventing financial losses caused by fraudulent ingredient substitution.
ELISA is widely employed in the food sector to comply with allergy labeling regulations. Because of its high sensitivity, even trace amounts of contamination, at the parts-per-million (ppm) level, can be detected. Another benefit is ELISA's ability to test items like egg whites, milk, and oils, which competing techniques like Polymerase Chain Reaction (PCR) cannot always effectively analyze due to the lack of DNA in highly refined products.
7. Analysis of Platelet Antibodies
Patients with autoimmune conditions such as immune thrombocytopenic purpura (ITP) and systemic lupus erythematosus (SLE) can be diagnosed by detecting platelet antibodies in their serum. In these assays, the enzyme alkaline phosphatase is conjugated to anti-human IgG to allow for detection.
In addition to being a cost-effective and less complicated approach for detecting platelet antibodies, ELISA has been proven to be more sensitive than other regularly used procedures, such as lymphocytotoxicity (LCT) and the platelet immunofluorescence test (PIIFT).
8. Vaccine Development
The ELISA method is a critical instrument in the development of innovative vaccines. In many instances, serum samples obtained from immunized humans or animal models are analyzed to detect specific antibodies directed against particular antigens. This is typically accomplished in carefully monitored trials where the host organism is administered specific antigens of interest.
However, choosing the proper assay for vaccine development is not always straightforward. Since the human immune response is complex, multi-dimensional, and polyclonal, selecting an assay that accurately reflects the disease pathology or the efficacy of the defense against infection remains a significant challenge.
Conclusion
The ELISA test has diverse applications across food science, medicine, pharmaceuticals, toxicology, drug monitoring, immunology, immunotherapy, and transplantation.
Due to the rising incidence of emerging diseases and the critical need for prompt diagnosis, ELISA protocols will be relied upon with increasing frequency. Because immunoassays provide stringent quality control and a reliable approach for rapid diagnostics, there will be an even greater need for ELISA kits within the pharmaceutical industry as it develops innovative vaccines and therapeutic formulations.
Furthermore, the food industry continues to expand allergy testing to meet growing health consciousness, while global health organizations utilize these assays to monitor viral infections and prevent potential pandemics.