ELISA: Introduction and types

Lab tests can help diagnose and treat a variety of health conditions, from minor ailments to life-threatening conditions. This emphasises the need to have quick, hassle-free, and accurate tests that have a wide range of applications. With the rapidly growing demand for the use of antibody-based assays in clinics, many of them now rely on antibodies or antigens that are bound to solid-phase supports, such as microtiter plates, or even beads of different kinds. Presently, one of the most popular and effective developments in antibody-antigen solid-phase support assays is the Enzyme-Linked Immuno-Sorbent Assay or ELISA. To have a wide range of applications, various types of ELISA have been developed. Let’s have a deeper look.

An assay is a set of procedures used to determine whether a compound or other molecule is present in a sample.

What is ELISA?

ELISA, or Enzyme-Linked Immunosorbent Assay, is a type of immunoassay. It is used to detect or measure the presence of a specific substance or molecule in a sample, such as those that may be involved in a disease. The test is based on the principle that antibodies can detect the presence of specific molecules by binding to them, which is then followed by a secondary antibody that detects the presence of the first antibody. 

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ELISA is based on the principle of using antibodies or even antigens that are covalently bound to enzymes.

Types of ELISA

Several variations of the basic ELISA assay have been developed. Each type can be used to detect the presence of antibodies or antigens (qualitatively). Let us discuss each type one by one:- 

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Indirect ELISA

To begin with, an indirect ELISA is a type of ELISA test.  In this case, an antigen is bound to a solid support. It is then detected using an indirect method, such as a secondary antibody. The antigen is often diluted and then directly added to the solid support. 

Indirect ELISA is seen to be more sensitive than the traditional direct ELISA method.

This type of ELISA is most effective to detect the presence of serum antibodies in the case of HIV-AIDS patients. 

Also read: Gel Electrophoresis- A Separation Technique – My Biology Dictionary

Sandwich ELISA

Sandwich ELISA is a technique used to test for the presence of a biological marker in a sample. The assay uses a capture antibody and a biotinylated or fluorescently labelled secondary antibody. In sandwich ELISA, the capture antibody binds to a specific target molecule in the sample, and the biotinylated or fluorescently labelled secondary antibody binds to the capture antibody. The resulting reaction product is then detected by chemiluminescence or enzyme-substrate complexes. The Sandwich ELISA is one of the most common ELISA formats used in the characterization of cytokinine in tissue culture supernatants.

Competitive ELISA

The competitive ELISA is similar to the sandwich ELISA in its basic components. However, rather than forming two layers sandwiching the antigen, the competitive ELISA forms only one layer with the antigen in the middle. The antigen is bound to a solid support, such as a microtiter dish, and then the support is coated with a solution of antibody. The antigen-antibody mixture is then added to the support. It is further allowed to compete with the original antigen for binding to the antibody.

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The amount of antibody bound to the solid support is a good indicator of the amount of antigen present. The solid support can be washed several times to remove excess antibodies, leaving just the amount of antigen that was originally bound to the solid support. In contrast to the sandwich ELISA, the competitive ELISA only requires a very small amount of antigen to be immobilised on a solid support. This allows for very sensitive measurements of very small amounts of antigen.

The enzyme-linked immunosorbent assay (ELISA) has been used for a wide range of diagnostic and forensic applications over the past half-century. Over the years, the techniques used to develop ELISAs have been refined and improved, and today’s lab technicians can choose from a wide variety of commercially available ELISA kits, which can be used to detect molecules ranging from antibodies to cancer cells. 

To summarize, there are many different types of ELISA that have evolved over the course of history. Some of them are better suited to specific applications than others. In the upcoming articles, we will cover more types of ELISA like Direct ELISA. 

To learn more about the diagnostic tests, keep reading at MBD.

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Watch ELISA here- elisa enzyme linked immunosorbant assay – YouTube