What is a SARS-CoV-2 Antigen test?
It is the detection of viral proteins or antigens and their presence is indicative of viral infection.
How is antigen testing used to diagnose or manage SARS-CoV-2 infection?
Antigen-based testing tests for the presence of SARS-CoV-2 protein antigen is a diagnostic test for SARS-CoV-2. A positive result indicates the presence of viral antigen(s) suggesting the patient’s symptoms are attributed to infection with SARS-CoV-2.
How does antigen testing compare to molecular or serology testing?
Molecular tests detect the presence of viral RNA through a nucleic acid amplification procedure. Like molecular tests antigen tests are diagnostic but do so by detecting the presence of viral proteins. Serology testing detects that presence of antibodies to SARS-CoV-2 and when present indicates a current or prior exposure and/or infection. Serology tests are useful to aid in diagnosis and surveillance.
How is antigen testing performed?
Antigen testing relies on antibodies specific for viral proteins to bind and a reporter system to recognize this binding. This is typically in the form of an ELISA or lateral flow-based approaches.
- What is neutralising antibody?
neutralising antibodies (nAb) are antibodies that protect the body from infection through the binding to the pathogen (such as bacteria and viruses) and directly interfere with their infectivity.
- Are all antibodies considered to be neutralising antibodies?
No. The presence of antibodies provides an indication whether an individual has been infected. However, not all antibodies have neutralising or “disarming” capacity.
Non-neutralising antibodies may bind to regions of a pathogen, but do not “disarm” the virus or prevent infection. Non-neutralising antibodies may have “protective” effects through different mechanisms, such as the recruitment of immune cells to “clear” the pathogen.
Neutralising antibodies, on the other hand bind to regions of the virus that “disarm” or prevent infection directly, without the involvement of immune cells.
- Does presence of neutralising antibody imply that an individual has protective immunity?
The presence of neutralising antibodies provides indications that an individual has protective immunity against infection.
In the context of COVID-19, it is important to note that there is currently little information on the level of neutralising antibodies sufficient to confer protective immunity to an individual and how long such protective immunity against COVID-19 may last in an individual.
- How does neutralising antibody inhibit infection?
Neutralising antibodies can inhibit infection through numerous mechanisms.
In the context of SARS-CoV-2 (the virus that cause COVID-19), one known mechanism of neutralising antibodies involve the blockage of interaction between the viral spike proteins (on the surface of the virus) and angiotensin-converting enzyme 2 (ACE2) receptors (on the surface of cells). This blockage prevents the virus from entry into the cell and prevents infection.
- What can the detection of Viral Spike/ACE2 blocking antibodies tells us?
In the context of SARS-CoV-2, the COVID-19 viral spike/ACE2 blocking antibodies may interfere with the ability of the virus to infect the target cells. The detection of these antibodies provides an indication of whether the tested individual might has developed protective immunity against COVID-19.
However, it is important to note that there is currently little information on the level of antibodies sufficient to confer protective immunity to an individual and how long such protective immunity against COVID-19 may last in an individual.
- How is a Viral Spike/ACE2 blocking antibodies test different from other antibody tests, such as the IgG/IgM rapid test?
When an individual is infected with an invading pathogen (such as SARS-CoV-2 that causes COVID-19), the body’s immune system will mount an immune response against such pathogen, thereby creating different types of antibodies (that is, proteins that fight infections), including IgG and IgM antibodies.
Most commercially available COVID-19 serology tests for COVID-19, including Biolidics’ COVID-19 Antibody Test Kit detect mainly IgG and/or IgM. Such serology tests are not able to determine if an individual has developed specific antibodies that bind to the SARS-CoV-2 virus and interfere with its ability to infect target cells of an individual. As such, they are currently used only as an assistive tool in the detection of COVID-19.
A Viral Spike/ACE2 blocking antibodies test detects the presence of specific types of antibodies which can bind to the SARS-CoV-2 virus and interfere with the interaction between the virus and its cellular receptor, ACE2, thereby preventing the virus from entering target cells to cause the infection.
Individuals who have developed such antibodies may potentially have acquired protective immunity against COVID-19.
ACE2 or "Angiotensin Converting Enzyme 2" are proteins on the surface of many types of cells (e.g, lung cells).
Viral Spike proteins from coronaviruses recognise and bind to ACE2 on the surface of target cells (e.g. lung cells), to facilitate the entry of the virus into the cells, resulting in an infection.1
Coronaviruses (CoV) are a large family of viruses. The Coronavirus can cause illness from the common cold to more severe diseases, such as Severe Acute Respiratory Syndrome (SARS).2
An infected individual may exhibit respiratory symptoms, cough, fever, shortness of breath and breathing difficulties. An infection can also cause severe acute respiratory syndrome, pneumonia, kidney failure and even death.
|Coronavirus disease 2019 (COVID-19) is an infectious disease caused by a newly discovered coronavirus, known as "SARS-CoV-2".3
|COVID-19 viral spike/ Angiotensin Converting Enzyme 2 ("ACE2") blocking antibodies
|Blocking antibody is a specific type of antibody that can prevent harmful substances (e.g. viruses, bacteria, or toxins) from binding to the cell. COVID-19 viral spike/ Angiotensin Converting Enzyme 2 ("ACE2") blocking antibodies can bind to Viral Spike protein thereby preventing its interaction with the cellular receptor ACE2 on a target cell surface.4
|IgG and IgM
The immune system makes different proteins to fight antigens, such as bacteria, viruses, and toxins. These proteins are known as antibodies. There are five subclasses of antibodies — IgA, IgG, IgM, IgD, and IgE.5
|Protective immunity is the ability to resist infection of an invading pathogen.5
|"SARS-CoV-2" or "Severe Acute Respiratory Syndrome Coronavirus 2" is a strain of coronavirus, identified in 2019, which causes coronavirus disease 2019 (COVID-19).3
|Serology tests are blood tests that look for antibodies in the blood. 6
|Viral Spike proteins
|Viral Spike proteins are a type of proteins present on the surface of the coronaviruses, which facilitate entry of the virus into cells, causing an infection.7
- Hoffmann, Markus, et al. "SARS-CoV-2 cell entry depends on ACE2 and TMPRSS2 and is blocked by a clinically proven protease inhibitor." Cell (2020).
- World Health Organisation. 2020. About COVID-19. [online] Available at: http://www.emro.who.int/health-topics/corona-virus/about-covid-19.html Accessed 12 July 2020.
- World Health Organisation. 2020. Naming The Coronavirus Disease (COVID-19) And The Virus That Causes It. [online] Available at: https://www.who.int/emergencies/diseases/novel-coronavirus-2019/technical-guidance/naming-the-coronavirus-disease-(covid-2019)-and-the-virus-that-causes-it Accessed 11 July 2020.
- Vinson, Valda. "An antibody defense against COVID-19." Science (2020): 1201-1203.
- Immunobiology: The Immune System in Health and Disease. 5th edition. Janeway CA Jr, Travers P, Walport M, et al. New York: Garland Science; 2001
- Centers for Disease Control and Prevention. 2020. Information For Laboratories About Coronavirus (COVID-19). [online] Available at: https://www.cdc.gov/coronavirus/2019-ncov/lab/serology-testing.html Accessed 12 July 2020.
- Belouzard, Sandrine, et al. "Mechanisms of coronavirus cell entry mediated by the viral spike protein." Viruses 4.6 (2012): 1011-1033.
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