Experts assume that antibodies generated by patients cured by COVID-19 protect them from a new infection by the coronavirus. But scientists believe that some people are able to fight COVID-19 even without these antibodies. Meet this mysterious protector, present in the body of about half of the world’s population.
In early March, Dutch researchers published on biorxiv.org an article warning that people who were never infected with COVID-19 could be immune to its pathogen. A month later, the article was published in Nature magazine, one of the most respected scientific publications in the world.
Biologists tested the human monoclonal antibody 47D11, detected during the epidemic of severe acute respiratory syndrome – called SARS. The SARS-CoV that causes SARS is similar to the pathogen that causes the current COVID-19 pandemic.
The antibodies introduced into the infected cells successfully neutralized the viral particles. The authors of the paper suggested that these antibodies could protect healthy people from infections and help infected patients heal from the virus.
At the same time, a similar antibody was discovered by an international team of scientists led by Swiss virologist Dora Pinto. In the blood of a SARS-infected patient in 2003, 25 antibodies were detected. However, only one of them neutralized the new coronavirus.
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Doctor analyzing examination of a patient with COVID-19 at the clinic of Moscow State University, Russia
Neutralization occurs because the antibody is able to recognize a portion of protein S on the surface of the viral particle that is characteristic of both pathogens. After recognition, the antibody binds to that particle, preventing SARS-CoV-2 from entering the cell.
In April, German researchers noted that sometimes it is not antibodies that protect against coronavirus – called humoral immunity, but T-lymphocytes – cellular immunity.
For them, macrophage cells swallow the pathogen and fragments of their proteins are deposited on its membrane. Fragments are recognized by T cells with the help of special receptors that, like antibodies, are immunoglobulins and specifically bind to antigens. This helps to ensure the immune response of the patient’s body.
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Graffiti by brothers Mikhail and Sergei Erofeev in honor of health workers, painted in the city of Krasnogorsk, Russia, May 15, 2020
Scientists have noted that some T cells are already able to respond correctly to SARS-CoV-2. Therefore, people who have these cells can deal with COVID-19 more easily or are classified as asymptomatic.
During the studies, scientists collected blood from patients infected with COVID-19 and healthy patients, who had no contact with those infected and had no antibodies to SARS-CoV-2. The lymphocytes found were isolated and stimulated with molecules corresponding to different fragments of the coronavirus S protein.
The result was that almost 30% of healthy volunteers had T cells capable of responding to a dangerous pathogen protein as the cause of COVID-19.
In addition, most of the time these cells recognized fragments similar to the protein S parts of other coronaviruses – as the cause of common colds, HCoV-229E.
Fortunately, SARS-CoV-2-responsive T cells have been found in most patients infected with COVID-19. Patients who lacked T cells usually faced more severe forms of the disease.
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Health agents conduct tests for COVID-19 on taxi drivers in Rio de Janeiro, June 15, 2020
According to the study authors, the data collected indicate a potential for SARS-CoV-2 reactive cellular immunity. In other words, people who previously had seasonal coronaviruses would already be immune to COVID-19.
This discovery could explain why children and young people are more tolerant to the disease. Often in crowded places, like kindergartens, schools and universities, this group is more likely to catch a common cold caused by coronavirus. Thus, the reactive immunity of young people and children can be strengthened.
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The hypothesis put forward by German scientists was confirmed about a month later by his American colleagues, who studied blood samples taken from patients from 2015 to 2018 – that is, before the spread of COVID-19.
In almost all biological materials, they found signs of specific cellular immunity, similar to those that are detected after SARS-CoV-2 infection.
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Children run down the street past an information board warning of the dangers of the new coronavirus in a slum in Nairobi, Kenya, June 3, 2020
In that case, the scientists referred to two types of immune cells – T-killer cells (CD8 cells) and T-helper cells (CD4 cells). The former recognize cells infected with the virus and destroy them – sometimes on their own, sometimes asking for help from other cells. The second group guarantees an increase in the number of T-killers and improves their combat capacity.
In about half of the samples taken three to five years ago, scientists identified specific CD4 cells that are now characteristic of people who have had COVID-19.
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Doctor performs test for new coronavirus in patient from Beccar region, Buenos Aires suburb, Argentina, June 17, 2020
In addition, CD8 cells, present in 70% of patients infected with the new coronavirus, were detected in 20% of patients whose samples were collected between 2015 and 2018.
This means that almost half of the healthy population on Earth may have immunity to COVID-19, indicate the authors of the work.
An additional study of blood samples obtained between 2015 and 2018 also indicated the presence of antibodies to the two most common coronaviruses, HCoV-OC43 and HCoV-NL63.
Therefore, people who have already been exposed to other coronavirus infections may be immune to SARS-CoV-2, which causes COVID-19. This discovery could explain the existence of asymptomatic cases of the disease.