Before the development of the ELISA, the only option for conducting an immunoassay was radioimmunoassay, a technique usingradioactively-labeled antigens or antibodies. In radioimmunoassay, the radioactivity provides the signal which indicates whether a specific antigen or antibody is present in the sample. Radioimmunoassay was first described in a paper by Rosalyn Sussman Yalow and Solomon Berson published in 1960.
Because radioactivity poses a potential health threat, a safer alternative was sought. A suitable alternative to radioimmunoassay would substitute a non-radioactive signal in place of the radioactive signal. When enzymes (such as peroxidase) react with appropriate substrates (such as ABTS or 3,3’,5,5’-Tetramethylbenzidine), this causes a change in color, which is used as a signal. However, the signal has to be associated with the presence of antibody or antigen, which is why the enzyme has to be linked to an appropriate antibody. This linking process was independently developed by Stratis Avrameas and G.B. Pierce. Since it is necessary to remove any unbound antibody or antigen by washing, the antibody or antigen has to be fixed to the surface of the container, i.e. the immunosorbent has to be prepared. A technique to accomplish this was published by Wide and Jerker Porath in 1966. In 1971, Peter Perlmann and Eva Engvall at Stockholm University in Sweden, and Anton Schuurs and Bauke van Weemen in The Netherlands, independently published papers which synthesized this knowledge into methods to perform EIA/ELISA.
The steps of "indirect" ELISA follows the mechanism below:-
A buffered solution of the protein antigen to be tested for is added to each well of a microtiter plate, where it is given time to adhere to the plastic through charge interactions.
A solution of non-reacting protein, such as bovine serum albumin, or casein is added to block any plastic surface in the well that remains uncoated by the protein antigen.
Next the primary antibody, generally in the form of serum is added, which contains a mixture of the serum donor's antibodies, of unknown concentration, some of which may bind specifically to the test antigen that is coating the well.
Afterwards, a secondary antibody is added, which will bind any antibody produced by a member of the donor's species (for example, an antibody produced in a mouse that will bind any rabbit antibody). This secondary antibody often has an enzyme attached to it, which has a negligible effect on the binding properties of the antibody.
A substrate for this enzyme is then added. Often, this substrate changes color upon reaction with the enzyme. The color change shows that secondary antibody has bound to primary antibody, which strongly implies that the donor has had an immune reaction to the test antigen. This can be helpful in a clinical setting, and in R&D.
The higher the concentration of the primary antibody that was present in the serum, the stronger the color change. Often a spectrometer is used to give quantitative values for color strength.
The enzyme acts as an amplifier; even if only few enzyme-linked antibodies remain bound, the enzyme molecules will produce many signal molecules. Within common-sense limitations the enzyme can go on producing color indefinitely, but the more primary antibody is present in the donor serum, the more secondary antibody + enzyme will bind, and the faster color will develop. A major disadvantage of the indirect ELISA is that the method of antigen immobilization is non-specific; when serum is used as the source of test antigen, all proteins in the sample may stick to the microtiter plate well, so small concentrations of analyte in serum must compete with other serum proteins when binding to the well surface. The sandwich or direct ELISA provides a solution to this problem, by using a "capture" antibody specific for the test antigen to pull it out of the serum's molecular mixture.
ELISA may be run in a qualitative or quantitative format. Qualitative results provide a simple positive or negative result (yes or no) for a sample. The cutoff between positive and negative is determined by the analyst and may be statistical. Two or three times the standard deviation (error inherent in a test) is often used to distinguish positive from negative samples. In quantitative ELISA, the optical density (OD) of the sample is compared to a standard curve, which is typically a serial dilution of a known-concentration solution of the target molecule. For example if your test sample returns an OD of 1.0, the point on your standard curve that gave OD = 1.0 must be of the same analyte concentration as your sample.
A sandwich ELISA. (1) Plate is coated with a capture antibody; (2) sample is added, and any antigen present binds to capture antibody; (3) detecting antibody is added, and binds to antigen; (4) enzyme-linked secondary antibody is added, and binds to detecting antibody; (5) substrate is added, and is converted by enzyme to detectable form.
A less-common variant of this technique, called "sandwich" ELISA, is used to detect sample antigen. The steps are as follows:
Prepare a surface to which a known quantity of capture antibody is bound.
Block any non specific binding sites on the surface.
Apply the antigen-containing sample to the plate.
Wash the plate, so that unbound antigen is removed.
Apply enzyme linked primary antibodies as detection antibodies which also bind specifically to the antigen.
Wash the plate, so that the unbound antibody-enzyme conjugates are removed.
Apply a chemical which is converted by the enzyme into a color or fluorescent or electrochemical signal.
Measure the absorbency or fluorescence or electrochemical signal (e.g., current) of the plate wells to determine the presence and quantity of antigen.
The image to the right includes the use of a secondary antibody conjugated to an enzyme, though technically this is not necessary if the primary antibody is conjugated to an enzyme. However, use of a secondary-antibody conjugate avoids the expensive process of creating enzyme-linked antibodies for every antigen one might want to detect. By using an enzyme-linked antibody that binds the Fc region of other antibodies, this same enzyme-linked antibody can be used in a variety of situations. Without the first layer of "capture" antibody, any proteins in the sample (including serum proteins) may competitively adsorb to the plate surface, lowering the quantity of antigen immobilized.Use of the purified specific antibody to attach the antigen to the plastic eliminates a need to purify the antigen from complicated mixtures before the measurement, simplifying the assay, and increasing the specificity and the sensitivity of the assay.
A descriptive animation of the application of sandwich ELISA to home pregnancy testing can be found here.
A third use of ELISA is through competitive binding. The steps for this ELISA are somewhat different than the first two examples:
Unlabeled antibody is incubated in the presence of its antigen (Sample)
These bound antibody/antigen complexes are then added to an antigen coated well.
The plate is washed, so that unbound antibody is removed. (The more antigen in the sample, the less antibody will be able to bind to the antigen in the well, hence "competition.")
The secondary antibody, specific to the primary antibody is added. This second antibody is coupled to the enzyme.
A substrate is added, and remaining enzymes elicit a chromogenic or fluorescent signal.
For competitive ELISA, the higher the sample antigen concentration, the weaker the eventual signal. The major advantage of a competitive ELISA is the ability to use crude or impure samples and still selectively bind any antigen that may be present.
(Note that some competitive ELISA kits include enzyme-linked antigen rather than enzyme-linked antibody. The labeled antigen competes for primary antibody binding sites with your sample antigen (unlabeled). The more antigen in the sample, the less labeled antigen is retained in the well and the weaker the signal).
Commonly the antigen is not first positioned in the well.Multiple and Portable ELISA (M&P ELISA)(ELISA Reverse in published papers)
A new technique (EP 1 499 894 B1 in EPO Bulletin 25.02.209 N. 2009/09; USPTO 7510687 in USPTO Bulletin 31.03.2009; ZL 03810029.0 in SIPO PRC Bulletin 08.04.2009) uses a solid phase made up of an immunosorbent polystyrene rod with 8-12 protruding ogives. The entire device is immersed in a test tube containing the collected sample and the following steps (washing, incubation in conjugate and incubation in chromogenous) are carried out by dipping the ogives in microwells of standard microplates pre-filled with reagents.
The advantages of this technique are as follows:
The ogives can each be sensitized to a different reagent, allowing the simultaneous detection of different antibodies and / or different antigens for multi-target assays;
The sample volume can be increased to improve the test sensitivity in clinical (saliva, urine), food (bulk milk, pooled eggs) and environmental (water) samples;
One ogive is left unsensitized to measure the non-specific reactions of the sample;
The use of laboratory supplies for dispensing sample aliquots, washing solution and reagents in microwells is not required, facilitating the deveopment of ready-to-use lab-kits and on-site kits.