Technique Tuesday: Enzyme-Linked Immunosorbent Assay (ELISA)

Sep 15, 2020
By Devina Thiono

Technique Name: Enzyme-Linked Immunosorbent Assay (ELISA)

Fun Rating:

Difficulty rating:  (high focus and concentration needed to make sure you know which sample goes where!)

What is the general purpose?

ELISA is a tool that can be used to identify the presence of antibodies or antigens in a patient’s sample.

An antigen is a foreign invader (such as bacteria, virus, pollen) that gets into the body and can cause an immune response. Some of the more common immune responses that we can see when our body is reacting to these invaders are inflammation, redness, fever and swelling. Antibodies are proteins produced by our immune cells (specifically, B cells) that recognize and bind to specific sites on antigens.

Why do we use it? 

ELISA is used to see if someone has a specific antigen/disease or has previously been exposed to an antigen. Antigen detection ELISA can be used to detect if the patient is currently infected with the bacteria, virus, etc. Antibody detection ELISA is used to determine if the person is currently or has previously been infected with the antigen in question.

How does it work?

Since we are still in the middle of the pandemic, let’s use the COVID-19 test as an example!

One of the tests used to determine if someone has had past exposure to COVID-19 is the antibody test. In the lab, this will be carried out using an ELISA.

There are many types of ELISA. Most of the time, ELISA is performed on a 96-well plate which allows for multiple samples to be tested at once. 

Image of a 96-well plate that is commonly used to run ELISA. Each well holds an individual sample.

Each of these 96 wells (pictured above) are coated with the antigen of interest (1) – in this case, a protein that is specific to SARS-CoV-2, the virus that causes COVID-19.

Then, blood serum from a patient, which may or may not contain antibodies against SARS-CoV-2, is added to the well (2)

Next, a secondary antibody that can recognize the human antibody is added (3). This secondary antibody is usually tagged with a label that can turn colorless substrate, added in the final step in ELISA (4), into a colored one. This colored substrate can then be measured. These steps are summarized in the figure below.

The plate is washed between each step, so that non-specific or excess antigens/antibodies will be washed away.This step reduces background noise so the final result is more clear.

If the transparent liquid in a well turns a color, this means that the sample in this well contains SARS-CoV-2 antibody. This suggests that the patient has previously been exposed to SARS-CoV-2. If a well stays clear, then the patient sample does not have SARS-CoV-2 antibodies (or contains very small, undetectable amounts) and therefore has not been exposed.

In addition to lab testing, ELISA is also used in over-the-counter pregnancy test and in-home HIV test.

Edited by Lane Scher and Taylor Tibbs