What is Immunity?

What is immunity?
The immune system protects the body from infection from pathogens, such as the novel coronavirus that causes COVID-19. If you want to understand testing for COVID and how vaccinations will help control the pandemic, understanding immunity is a good place to start.

What is the immune system?
The immune system is made up of two primary branches, the innate and the adaptive immune system [1]. These two branches differ in the time it takes them to respond to potential threats.

The innate immune system is the first line of defence and consists of several barriers to infection. The skin provides the first physical barrier. If pathogens break through the surface through wounds or other routes, immune cells in the skin quickly respond to pathogens causing swelling and redness. This initial response is effective at destroying most pathogens that enter the skin before they can produce significant infections. The innate immune system does not target specific disease antigens (substances that cause an immune response) and is not responsible for providing immunity.

That is where the adaptive immune system comes in; a crucial part of the adaptive immune system is immune memory. Immune memory is a powerful method of disease prevention as the adaptive immune system reacts more rapidly to pathogens it has previously encountered.  This response usually prevents symptoms upon re-exposure, meaning you do not suffer from some infections more than once.

The adaptive immune system provides this memory by producing antibodies. Antibodies are proteins that recognise substances from pathogens (antigens). Some types of antibody target the pathogen for destruction and act more specifically and effectively, although more slowly, than the innate immune system. Other antibodies provide immunity against future infections of the same pathogen by providing a 'memory' of a pathogen.

What is the difference between natural and induced immunity?
Development of natural immunity requires infection with a pathogen and progression of the associated disease. In comparison, vaccination induces the immune system to develop immunity to a pathogen before you are exposed to it, protecting you against developing the disease in the future. 

Vaccines typically contain antigens associated with specific pathogens, along with substances called adjuvants, which help to stimulate the immune system. Some vaccines, such as the influenza vaccine, contain inactivated or weakened forms of the pathogen. 
The purpose of vaccines is to stimulate the immune system's natural process to produce antibodies against specific pathogens. 

Antibodies that enable immune 'memory' are called IgG antibodies. Successful, widespread vaccination can prevent the spread of a disease within a population by preventing its transmission from person to person, even eradicating diseases such as smallpox [2]. 

Does the presence of IgG antibodies mean I have immunity?
For many infectious diseases, a simple blood test can determine whether you are likely to be immune.  The presence of IgG antibodies in the blood against a specific pathogen typically confers immunity. 

However, some pathogens, such as the influenza virus are constantly changing. Different strains of the virus develop when the pathogen's genetic material changes. When a new strain develops, you may not have antibodies against the new strain's antigens. So while you may have immunity to one strain of the virus, you may still be infected by new strains to which your body is not immune.

As COVID-19 is a new infection, there is not enough research yet to be confident that antibodies produced against the virus will confer permanent immunity. Determining whether permanent immunity develops is critical to understanding whether re-infection is possible [3]. Initial studies have shown promising results with short-term immunity in mammals [4]. 

Significant efforts are being made around the globe to produce a vaccine against the disease, and some human trials are already underway [5]. Developing a vaccine will enable us to induce immunity and protect many people from COVID-19.

Can we use antibodies to test for previous infections?
The presence of specific IgG antibodies against the novel coronavirus can be detected in the blood. This means you can take a simple finger-prick blood test to check for previous infections with diseases such as COVID-19. If you would like to know whether you have had COVID-19, you should take our "Coronavirus (COVID-19) IgG Serology Antibody Blood Test". 

The total levels of different antibodies IgG, IgM and IgA can also be used to check for an active infection. If you would like to check for an active infection, you can use our "Immunoglobulins IgG IgM IgA" test.

References
1.    Murphy, K. Janeway’s Immunobiology, Ninth Edition. Yale J. Biol. Med. 89, (2016).
2.    Strassburg, M. A. The global eradication of smallpox. Am. J. Infect. Control 10, 53–59 (1982).
3.    Randolph, H. E. & Barreiro, L. B. Herd Immunity: Understanding COVID-19. Immunity 52, 737–741 (2020).
4.    Bao, L. et al. Lack of Reinfection in Rhesus Macaques Infected with SARS-CoV-2. bioRxiv 2020.03.13.990226 (2020). doi:10.1101/2020.03.13.990226
5.    Ahn, D.-G. et al. Current Status of Epidemiology, Diagnosis, Therapeutics, and Vaccines for Novel Coronavirus Disease 2019 (COVID-19). J. Microbiol. Biotechnol. 30, 313–324 (2020).