Antibodies are molecules produced by our immune cells against antigens (foreign bodies) and have the property of binding to the antigens in a specific manner. These specific antibodies are easy to identify and we can get indirect proof of the presence of antigens.

A correct diagnosis is a key factor towards a better treatment of any ailment and recent advances in the field of immunodiagnostics have accelerated the treatment procedure. Scientists around the world are working constantly in developing more specific, more sensitive, and more cost-effective procedures of diagnosis.

Types of Immunodiagnostics

Radioimmunoassays (RIA), developed in the 1950s, were the earliest immunoassays and are one of the most sensitive methods that rely on radiolabeling. Nonetheless, their use has been precluded owing to the health risk due to radioactive exposure.

ELISAs replaced RIAs and are one of the most common and reliable diagnostic tools until now. The antigen-binding antibody is linked with an enzyme that gives a color reaction in the presence of a substrate. The market is constantly changing and researchers are developing new methods of immunodiagnostics that are furthermore accurate and cost-effective as discussed below:

Chemiluminescent immunoassays (CLIAs) couples immune reaction with chemical reactions that emit a photon (light) that can be easily detected.

Fluorescent immunoassays (FIAs): here antibody-binding antigen is tagged with a fluorescent chemical compound that can re-emit light upon excitation with a particular wavelength of light. Its utility may be further enhanced by the use of flow cytometers.

Lateral flow assays (LFAs) or Lateral Flow Test is a simple diagnostic device to confirm the presence or absence of a pathogen or some other entities that may be present in water, foodstuff, urine, or blood sample. It typically contains a line to confirm the validity of the kit and other line/s to confirm the presence/absence of the analyte. The most common example is the pregnancy test kit. They can be used at home i.e point of care testing and require the least training. They have wide applications in health, food and agriculture, and the environment sector.

Application of Immunodiagnostics

Immunodiagnostics has applications (but not limited to) in the following fields

Immunodiagnostics in oncology

Tumor-associated antigen or antibodies are the primary targets for cancer detection. This is also important in finding tumor relapse. Many cancer markers have been identified which can help in the accurate and early diagnosis of breast cancer, renal neoplasm, oral cancer, pancreatic cancer, and many others. For example, recently OPKO Health Inc. has got the US FDA approval to use microfluidics approaches, for prostate-specific antigen (PSA) testing to increase the accuracy of prostate cancer diagnosis and reduce unnecessary biopsies.

Immunodiagnostics in infectious diseases, Allergy, and autoimmune diseases

Polymerase chain reaction (PCR) is the best choice for the detection of an active infection. However, in many cases such as chronic infection (when a parasite goes in the dormant phase), immunoassays are more reliable. One limitation of ELISA is that it has low sensitivity in the case of the low analyte. This has been overcome by the use of IPCR or immuno-PCR which combines the amplification power of PCR and the sensitivity of ELISA. Here the primary immobilized antibody binds with a secondary antibody tagged with a nucleic acid capable of amplification (Fig.2.). Antibody-based approaches are now very helpful in the diagnosis of autoimmune diseases and allergic reactions. These methods also reduce the pain of invasive biopsies.

Immunodiagnostics in environmental pollution

The growing concern over environmental pollution and its link to human diseases have warranted the authorities to implement stringent policies. Immunodiagnostic tools have served greatly in this regard by detecting environmental contaminants like Dichlorodiphenyltrichloroethane(DDT), Pentachlorophenol (PCP), polychlorinated biphenyls (PCB), polycyclic aromatic hydrocarbons (PAH), organophosphorus pesticides, explosives, biological warfare materials, etc.

Immunodiagnostics in Food Technology

Immunodiagnostics has been widely used to monitor contaminants in different foodstuffs such as pathogenic microorganisms in food, staphylococcal enterotoxin B, melamine, and antibiotics in milk, wheat proteins in milk powder and pesticides in fruits, vegetables, and grains.

Immunodiagnostics in proteomics study

Proteomics deals with the protein profile produced by our cells. Mass spectrometry (MS) has been a useful tool in proteomics research. MS coupled with immunoassay provides extraordinary ability to perform targeted proteome screening. This is called mass spectrometric immunoassays (MSIAs) which is very sensitive and fast and can screen several proteins at a time.  

Next-generation immunodiagnostics 

Rapid advances in the Interdisciplinary field of biomedical instrumentation and immunodiagnostics is enabling us to step towards the next-generation immunodiagnostics. The major revolutionizing forces have been the microfluidics and magnetic beads based immunoassays.

Microfluidic assays

The precise control and manipulation of mass transport of minute quantity of fluids confined in a sub-millimeter space through capillary movement governs the principle to microfluidics and has wide application in physics, chemistry, and biotechnology. It has applications in paper-based immunodiagnostics like portable glucose detection and environmental testing.

Magnetic immunoassays

Magnetic beads are nanosized magnetic particles glued together with polymers that show the properties of superparamagnetism in the presence of an external magnetic field. These beads can be conjugated to antibodies for immunobased assays. These are superiority over ELISA or IFA. Magnetic beads based immunoassays are more sensitive, give no false-positive results, are easy in separation and automation. This makes it the next choice of researchers and clinicians.

Lab-On-a-Chip (LOC)

As the name suggests, this is a compact device of the dimension of a few square millimeters to a few square centimeters that can perform several laboratory functions in a single integrated circuit or chip. They are a part of the microelectromechanical system and involve microfluidics. The ability to handle picolitres of the sample, accurate automation, and high throughput make LOCs the future of diagnosis.

According to the latest research report[1] , the global immunodiagnostics market size was valued at USD 15,777.5 Million in 2017. This is projected to reach USD 22,732.7 Million by the end of 2025 with a CAGR of 4.7%. Increased applications of immunodiagnostics in diverse areas like disease burden, environment pollution, and food safety is attracting huge market and will provide an opportunity to biotechnologists skilled in this field in both R&D and production industries that manufacture diagnostic kits. 

[1]https://www.fortunebusinessinsights.com/industry-reports/immunodiagnostics-market-100444. accessed on 19^th July, 2020.

*All the diagrams are self drawn by the author, Dr. Manoj Kumar Singh using Microsoft Paint 3D software.