It is assumed that antibodies to coronavirus in the blood of survivors of COVID-19 protect against re-infection. Some experts even propose to introduce special "immune passports" that allow those who have been ill to move freely around the world. However, as scientists have found out, there are people whose body can cope with a dangerous pathogen without it.
Prepared immunity
In early March, an article by Dutch researchers was posted on the biorxiv.org website that those who have never had COVID-19 may have immunity against its pathogen. A month later, this work was published by Nature, one of the most respected scientific journals in the world.
Biologists tested human monoclonal antibodies 47D11, obtained during an epidemic of severe acute respiratory syndrome - the so-called atypical pneumonia. The SARS-CoV that causes it is similar to the current pathogen.
Antibodies injected into infected cells successfully neutralized viral particles. The authors of the work suggested that these antibodies can protect healthy people from infection, and help patients get rid of the virus.
Around the same time, a similar antibody was discovered by an international team of scientists led by Swiss virologist Dora Pinta. They counted 25 antibodies in the blood of a patient who had recovered from SARS in 2003, but only one neutralized the new coronavirus. It recognizes on the surface of the viral particle a region of the S-protein that is characteristic of both pathogens, binds to it and prevents SARS-CoV-2 from entering the cell.
SARS-CoV viruses (left) and SARS-CoV-2 (right) under a microscope. According to several studies at once, antibodies to the first, available in people who have had atypical pneumonia, prevent SARS-CoV-2 from entering cells and spreading throughout the body.
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Cells remember
In April, German researchers said that sometimes it is not antibodies that protect against coronavirus - the so-called humoral immunity, but T-lymphocytes - cellular immunity. The bottom line is this: macrophage cells eat the pathogen, and fragments of its proteins are then placed on their membrane. They, in turn, are recognized by T cells using special receptors, which, like antibodies, are immunoglobulins and bind specifically to antigens. This promotes the immune response.
As it turned out, some T cells already know how to properly respond to SARS-CoV-2. That is why such people are easier to tolerate COVID-19 or are generally asymptomatic.
Scientists took blood from COVID-19 patients and healthy people who had not been in contact with infected people and without antibodies to SARS-CoV-2. Lymphocytes were isolated, which were then stimulated with molecules corresponding to different fragments of the S-protein of the coronavirus
It found that nearly 30 percent of healthy volunteers have T cells that respond to a protein from a dangerous pathogen. Moreover, they most often recognized those fragments of it that were similar to parts of the S-protein of other coronaviruses - for example, the HCoV-229E that causes the common cold. Also, antibodies to this type of viruses were detected in their blood.
In addition, SARS-CoV-2 responsive T cells have been found in most COVID-19 patients. Those who did not have them, as a rule, were more severely ill.
According to the authors of the study, their findings indicate the potential for cross-reactive cellular immunity to SARS-CoV-2. In other words, people who have previously had seasonal coronaviruses (it is believed that they cause up to 20 percent of common SARS) are already immune to COVID-19.
This, in particular, can explain why children and young people are easier to tolerate the disease. They are more likely to visit crowded places (the same kindergartens, schools and universities), which means that they are more likely to catch the common coronavirus cold, which, in turn, provides cross-reactive immunity.
Had been ill, then protected
The assumption of German scientists a month later was confirmed by their American colleagues who studied blood samples taken from patients from 2015 to 2018 - that is, when they did not know about COVID-19. In almost all biological materials, signs of specific cellular immunity were found, similar to those that appear when infected with SARS-CoV-2.
We are talking about two types of immune cells - killer T cells (CD8 cells) and helper T cells (CD4 cells). The former recognize cells infected with viruses and destroy them - sometimes on their own, sometimes calling on colleagues for help. The latter increase the number of T-killers and enhance their response to a specific pathogen.
In about half of the samples taken three to five years ago, scientists have identified specific CD4 cells that are now characteristic of people who have had COVID-19. In addition, 20 percent of them also contained CD8 cells, which are found in 70 percent of patients with the new coronavirus. This means that almost half of the healthy population of the Earth may well have immunity to a new disease, the authors of the work indicate.
An additional study of blood samples obtained in 2015-2018 showed antibodies to two of the most famous coronaviruses that infect humans, HcoV-OC43 and HcoV-NL63. In other words, people who have previously had other coronavirus infections have acquired immunity to SARS-CoV-2. This probably explains the asymptomatic COVID-19.