All about ELISA

What is it?

Antibodies recognize and bind to specific antigens, such as peptides, proteins and hormones. The enzyme-linked immunosorbent assay, better known as ELISA, exploits this antigen–antibody specificity to detect and measure, with the help of enzymes, the presence of proteins in samples of unknown composition and concentrations.

How does it work?

In a plate-based ELISA, researchers immobilize an antigen in a microwell plate. Next, they add antibodies that recognize the antigen at sites called epitopes. These antibodies are special in that they are coupled to specific enzymes. The researchers then treat the antigen–antibody complex with appropriate chromogenic substrates that are catalyzed by the antibody-conjugated enzyme, resulting in a color change. Finally, this color change is read and analyzed by a microplate reader.

Researchers have tinkered with this basic principle of ELISA to develop more sensitive variants. The sandwich ELISA, for example, uses of a pair of antibodies — a capture antibody and an enzyme-linked detector antibody — to recognize two separate epitopes on the same antigen. The antibody pairs in a sandwich ELISA significantly reduce background signals and make antigen detection a lot more specific than a conventional ELISA.

The recently patented multiple and portable ELISA, known as the M&P ELISA, facilitates simultaneous detection of multiple antigens. The M&P ELISA has a central protruding rod coated with eight to 12 immunosorbent pins, first immersed in the antigen and then dipped into reagent-filled microwell plates.

How did it come about?

In 1960, Solomon Berson and Rosalyn Yalow at the Bronx Veterans Administration Medical Center described an immunoassay they developed to measure insulin in human blood plasma using radioactivity. Yalow went on to win the 1977 Nobel Prize in medicine for the work (Berson died in 1972 and couldn’t be awarded the prize posthumously).

The technique garnered much attention, but it was accompanied by concern over the long-term effects of using radiation. The idea of replacing radiation with less hazardous substances was born out of a need to maximize safety while minimizing costs. Though initially met with immense skepticism, the concept of using enzymes as a reporter label started to gain popularity.

In 1971, three groups independently and simultaneously published their work proving the feasibility of ELISA as an alternative to radioactively labeled immunoassays: Eva Engvall and Peter Perlmann in Stockholm University measured immunoglobulins in rabbit serum, Anton Schuurs and Bauke van Weeman from the Organon Pharmaceutical laboratories in the Netherlands quantified human gonadotropin hormone in urine samples, and Stratis Avrameas and Guilbert at the Pasteur Institute in France measured serum immunoglobulin levels.

What are its applications?

In the 1970s, the American pharmaceutical company Abbot Laboratories first developed solid-phase radioimmunoassay kits to detect hepatitis B. Since then, ELISA has become a powerful diagnostic tool and now is used routinely to measure serum antibody concentrations against various toxins and pathogens, such as celiac disease, mycobacterium tuberculosis and the influenza virus.

In 1976, Dennis Bidwell and Alister Voller from the Institute of Zoology in London introduced the first sensitive microplate assay to screen for viral infections, including malaria. This microplate format since has been adapted to detect other viruses – with its best-known application being the screening test for human immunodeficiency virus titers in patients.

ELISA also has affected our daily lives in the form of home pregnancy tests, ovulation tests, over-the-counter bacterial infection kits and drug tests.