One of the keys to combating COVID-19 and the flu is to understand how our immune system responds to different viruses. Yasser Aldhamen, assistant professor, Microbiology and Molecular Genetics, at MSU’s College of Osteopathic Medicine, answers questions about our immune system.
Briefly, how does our immune system work?
Our immune system has a complex network of cells and proteins that recognize specific pathogenic structures present on the surface or inside the invading pathogens. Upon recognition of these structures by specific immune receptors, called pathogen recognition receptors or PRRs, our immune cells produce a diverse set of proteins and immune mediators that destroy these pathogens and develop long-live memory immune responses.
To answer this question, let’s first discuss the two types of the immune system. Our immune system is composed of two arms, the innate and adaptive immune systems. The innate immune system acts fast (in minutes) after it recognizes a pathogen and, in most cases, eradicates the invading pathogens. During this process, the cells of the innate immune system, and their derived immune mediators/proteins, also activate the cells of the adaptive immune system which then develop memory immune responses toward these pathogens. Therefore, upon reinfection, the intensity of the innate immune system remains the same. In contrast, the adaptive immune response is much stronger than the initial exposure to these pathogens.
Remaining healthy despite a frequent exposure to a large number of pathogens on a daily bases is one sign of having a strong immune system. Another sign of a strong immune system is the natural response to vaccination. Most vaccines include immune stimulants (called adjuvants), which are needed to activate the innate immune system and make vaccines work better. Activation of the innate immune system is commonly associated with a short-term increase in body temperature that usually lasts 24 hours. This means that developing a mild fever following vaccination indicates a person has a properly functioning immune system.
Yes, eating certain foods can strengthen our immune system, and contribute to a better control of infection. However, there is no proof that a certain supplement can cure or prevent disease. We talked above about the induction of the immune system during infection or in response to vaccination. During this phase of the immune response, the cells of our immune system need nutrients to proliferate and synthesize proteins that contribute to pathogen eradication and the induction of memory immune responses.
Vaccines are used to trick our immune system to develop memory immune response against the administered, nonpathogenic antigens. Vaccines are of different types and contain various immune stimulants (substances that are similar to pathogenic structures and are known to activate specific innate immune receptors). The most common form of vaccines is the attenuated pathogens (either viruses or bacteria), however, recent development in the vaccinology field resulted in the development of newer forms of vaccines, such as mRNA- and DNA-based vaccines.
In order for the vaccine to work, it needs to contain a specific adjuvant (such as aluminum-based adjuvant, which has been used since 1930 with an excellent safety profile). Vaccine adjuvants activate the cells of the innate immune system and help them work better. The most important cells of the innate immune system that respond to vaccines are called dendritic cells. These cells take-up/engulf the vaccine and present fragments from the antigen to T and B cells that then develop memory immune responses to the administered vaccine antigen.
How has Covid-19 affected our immune system – at least what do we know at this point?
SARS-CoV2 virus has been shown to dramatically affect many cells and pathways in our immune system. The increased mortality from SARS-CoV2-infected patients has been associated with increased inflammatory responses, known as a cytokine storm, which lead to multiple organ failure and significantly contribute to mortality in COVID-19 patients.
The innate immune response to SARS-CoV-2 is not fully understood, but recent findings indicate increased total numbers of innate immune cells in the lungs of infected patients, and high serum levels of multiple proinflammatory cytokines.
Despite the ability of the innate immune system to recognize and respond to SARS-CoV2 infection, recent research indicate that SARS-CoV2 evade our immune system by many mechanisms, such as dysregulation of innate immune cell functions and suppression of the antiviral immune response. In addition to inhibiting the innate immune responses, SARS-CoV2 also alter the adaptive immune cell functions, including depletion of T lymphocytes. A reduced number of T lymphocytes is associated with a worse COVID-19 disease outcome. Furthermore, the remaining T cells have a dysregulated function.
By all accounts, immunity to Covid-19 lasts only a short time – possibly – a few months. Why?
The exact mechanism is not known at the moment, but information from patients that were infected with the other coronavirus (SARS) during 2002 and 2003 showed that antibody responses – immunity – can last up to two to three years postinfection. Similarly, memory T cells toward SARS have been detected even 11 years after SARS infection. So, it may be that patients who get infected by COVID-19 develop a similar memory, but it’s hard to know this right now.
The most recent data from a clinical trial performed in the UK suggest that antibody and cellular immune responses can last at least six months after SARS-CoV2 infection. Although some studies suggest that immune responses to SARS-CoV2 are not strong, however, these responses (in particular neutralizing antibodies) to the virus were still detectable and might provide a long-term protection.
Critically, multiple studies found that memory B cells (which produce neutralizing antibodies) to the virus persist after a mild COVID-19 infection. This is important because upon reexposure, these cells can induce a quick and effective immune response against the virus.
If immunity wanes, what does this mean for a vaccine? Will we have to get one yearly?
We don’t know the answer to this question at the moment. However, a vaccine that is effective and able to induce higher neutralizing antibody and T cell memory responses might protect for a few years. Although the coronavirus doesn’t mutate as quickly as the influenza virus, some scientists think that people might need to take a booster every two to three years. Also, others have suggested an annual vaccination.
Personally, I think we will need to take the vaccine every year, since even after exposure, a long-lasting immunity to the virus might not be developed.