The virus that causes COVID-19 is easily transmitted from person to person. But sometimes it happens this way: in the family one is sick, and the other, caring for him, is not. Why don’t some people get infected even with close contact with a sick person?
Another mystery: many, after suffering COVID-19, do not have protective antibodies. Do they develop immunity in this case? And are there people who are resistant to this infection?
The answers to these and other questions are given by a study by a group of employees of the National Medical Research Center of Hematology of the Ministry of Health of the Russian Federation.
Cells are virus killers
Their article, just published in the journal Immunity, one of the most respected immunological journals in the world, has already attracted the attention of foreign colleagues. One of the authors of the article tells about his work, k. b. D., Head of the Laboratory of Transplant Immunology, National Medical Research Center of Hematology, Ministry of Health Grigory Efimov:
“We are studying, in particular, T-lymphocytes, which play an important role in the antiviral immune response. Having engaged in their research with COVID-19, we discovered a number of interesting facts.
For example, it was found that T-lymphocytes that recognize this virus can be found not only in those who have been ill, but also in people who have not come into contact with the virus in any way. Separately, we studied a group of people who were in close contact with patients with COVID-19, but not only did not have any symptoms of the disease, but also did not develop antibodies to this virus. It turned out that many of them have T-lymphocytes that recognize the coronavirus and probably provide them with protection from it.
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It should be explained here that the acquired immunity system consists of two parts. The first is represented by antibodies – these are special protein molecules (immunoglobulins). They are produced by the body in response to an attack by viruses or bacteria and are found in the blood plasma. Such immunity is called humoral, from the Latin word “humor” – liquid. And today, when they talk about COVID-19, it is mainly about them. The presence of antibodies in the blood is used to confirm the diagnosis. Also, these antibodies are formed after vaccination. In addition, the blood serum of those who have been ill, containing a large amount of protective antibodies, can be used in the treatment of COVID-19.
The second part of the acquired immunity is cellular. It is represented by special cells – lymphocytes. Among which there are T-lymphocytes (sometimes they are simply called T-cells). Some of the T-lymphocytes, the so-called killer T cells, kill virus-infected cells and thereby prevent the production of new viral particles. Other T cells (called helper T cells) are needed by the immune system to produce antiviral antibodies.
T-lymphocytes can do more than just fight off an infection. They keep the memory of the collision of the organism with the virus for a very long time, for years. If the virus enters the human body again, it is thanks to the immune memory that the infection will not develop or will proceed much easier.
Immunity in reserve
During the study, we estimated the number of antibodies and T cells in 3 groups of donors. The first included those who had been ill with COVID-19, the second and third groups included only healthy donors who did not have coronavirus. We took blood from donors from the second group in the spring of 2020, when the pandemic was already in full swing. The donors from the third group were people who donated blood to the blood bank of our Hematology Center before 2019, that is, when the world had not yet faced COVID-19.
The results were interesting. Firstly, not all COVID-19 patients have antibodies – in some of the recovered, the immune response is provided only at the expense of T cells. Apparently, they are quite enough to protect the body.
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Secondly, and this is the most interesting thing, sometimes the T-cell response to coronavirus is observed in people who have not had it. Moreover, we observed a T-cell response in donors from both groups. At the same time, healthy donors whom we recruited already during the pandemic, on average, had a higher level of T-cell response than those who donated blood before 2019.This is probably due to the fact that some healthy people in one way or another had contact with a virus without knowing it.
But how to explain that T-lymphocytes were also found in those who could not have had contact with the pathogen COVID-19? Most likely, this is the result of the so-called cross-immunity. Many seasonal colds, in particular SARS, are also caused by viruses from the coronavirus family. Some of them are probably similar in terms of T-lymphocytes to the virus that causes COVID-19. Therefore, T cells are prepared in advance to fight it. Perhaps this makes cross-immune people immune to COVID-19.
Thirdly, we found out which parts of the coronavirus are recognized by T-lymphocytes. These data can be used to create a test that evaluates T-cell immunity. The test will make it possible to understand whether a person has suffered COVID-19 or not, even in the absence of antibodies. This is important, because we know that antibodies do not always arise after infection, especially in those with mild or asymptomatic infection. Today, in such cases, the diagnosis cannot be confirmed.
We have already developed such a test. For now, we are using it for scientific purposes, but we expect that by the beginning of 2021 it will be registered for clinical use as well.
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