Serological test

Subject:Science and tech

Context:

Various serological tests to detect antibodies against novel coronavirus (SARS-CoV-2) could improve diagnosis of COVID-19 and be useful tools for epidemiological surveillance.

Concept:

ELISA

• ELISA (enzyme-linked immunosorbent assay) is a plate-based assay technique designed for detecting and quantifying peptides, proteins, antibodies and hormones.
• In an ELISA, an antigen must be immobilized to a solid surface and then complexed with an antibody that is linked to an enzyme.
• Detection is accomplished by assessing the conjugated enzyme activity via incubation with a substrate to produce a measureable product. The most crucial element of the detection strategy is a highly specific antibody-antigen interaction.

Types:

• ELISAs can be performed with a number of modifications to the basic procedure: direct, indirect, sandwich or competitive.
• The key step, immobilization of the antigen of interest, can be accomplished by direct adsorption to the assay plate or indirectly via a capture antibody that has been attached to the plate.The antigen is then detected either directly (enzyme-labeled primary antibody) or indirectly (enzyme-labeled secondary antibody). The detection antibodies are usually labeled with alkaline phosphatase (AP) or horseradish peroxidase (HRP).
• A large selection of substrates is available for performing the ELISA with an HRP or AP conjugate. The choice of substrate depends upon the required assay sensitivity and the instrumentation available for signal-detection (spectrophotometer, fluorometer or luminometer).

Lateral flow immunoassays (LFIAs)

• Lateral flow immunoassays (LFIAs) are a staple in the field of rapid diagnostics.
• These small handheld devices require no specialized training or equipment to operate, and generate a result within minutes of sample application.

• They are an ideal format for many types of home test kits, for emergency responders and for food manufacturers and producers looking for a quick evaluation of a given sample.
• The most commonly known type of lateral flow rapid test strip is the pregnancy test

Chemiluminescent immunoassays (CLIA)

• Chemiluminescent immunoassay is a variation of the standard enzyme immunoassay (EIA), which is a biochemical technique used in immunology.
• During EIA the process uses enzyme labeled antibodies and antigens to detect the small biological molecules required. The technique makes use of the basic immunology concept that an antigen binds a specific antibody. Such antigen molecules, which can be identified in a fluid sample, include molecules such as peptides, hormones and proteins.
• The enzymes used in chemiluminescentmicroparticle immunoassay convert a substrate to a reaction product, which emits a photon of light instead of developing a particular colour.

Immunoglobulin class

Antibodies are proteins made by the immune system to fight antigens, such as bacteria, viruses, and toxins.The body makes different immunoglobulin to combat different antigens.

The five subclasses of antibodies are:

• Immunoglobulin A (IgA), which is found in high concentrations in the mucous membranes, particularly those lining the respiratory passages and gastrointestinal tract, as well as in saliva and tears.
• Immunoglobulin G (IgG), are involved in the secondary immune response (IgM is the main antibody involved in primary response). IgG can bind pathogens, like for example viruses, bacteria, and fungi, and thereby protects the body against infection and toxins. Comprising up to 80% of the antibodies found in the human body, IgG is the smallest, yet most abundant human antibody, and that of other mammals. IgG can be found in all bodily fluids, and is the only antibody that can protect a foetus by passing through the mother’s placenta.
• Immunoglobulin M (IgM), is the largest antibody, and it is the first antibody to appear in the response to initial exposure to an antigen. B-cells create IgM antibodies as a first line of defense. Their large size gives them excellent binding avidity, and can pick up trace amounts of infection to mark for recognition by phagocytes. IgM is primarily found in serum and due to its size, it cannot diffuse well, and is found in the interstitium only in very low quantities.
• Immunoglobulin E (IgE), which is associated mainly with allergic reactions (when the immune system overreacts to environmental antigens such as pollen or pet dander). It is found in the lungs, skin, and mucous membranes.
• Immunoglobulin D (IgD), which exists in small amounts in the blood, is the least understood antibody.