Those who develop Covid-19 in a mild form may have already "known" other coronaviruses

Those who develop Covid-19 in a mild form may have already known other coronaviruses

A study found many more memory T cells in the blood of patients with mild Covid-19 symptoms, specific to the antigenic sequences Sars-Cov-2 shares with other coronaviruses

(illustration: Getty Images) Those who contracted Covid-19 but in a mild form, perhaps had already had "close encounters" with other coronaviruses. The hypothesis comes from a team of researchers at Stanford University School of Medicine, who on average found in the blood of Covid-19 patients with mild symptoms more "traces" of previous infections by common cold coronaviruses, which however they share some protein sequences with their more feared cousin Sars-Cov-2. The study, published in the journal Science Immunology, would thus explain why children, who in the first years of life are "immersed" in common seasonal viruses, generally do not get Covid-19 or show mild symptoms, even if they are equally likely to contracting Sars-Cov-2 of an adult.

Not just antibodies

Although much attention has been given to the development of antibodies against pathogens, this is not the only form of defense of our body. And thank goodness, because - as the head of research Mark Davis explains - viruses such as Sars-Cov-2 are able to evade antibodies with relative ease by "hiding" the antigens, the target protein sequences.

A ' another very effective defense weapon of the immune system are killer T lymphocytes, an evocative name for these cells that have the task of probing the organism in search of foreign protein sequences exposed on the surface of other cells and, once recognized, multiply quickly to wipe out anything with the same mark. For the system they use to identify the enemy, killer T cells aren't that easy to fool.

It doesn't stop there. Among the many "daughter" cells produced by the killer T lymphocyte that first recognized an intruding sequence, there will be some a little more "peaceful" which, instead of attacking and destroying, take note of the characteristics of the pathogen and remember it throughout their long life. So if it does recur they will be ready to activate a much faster defense response and smother the threat in the bud.

Why don't some people get Covid-19?

Davis and his collaborators have speculated that the reason why some people, despite having contracted Sars-Cov-2, have no symptoms or manifest a mild form of Covid-19 could reside in the arsenal of killer T lymphocytes at their disposal.

To verify this, they first analyzed the antigen sequences of Sars-Cov-2 and other common coronaviruses colds. They selected 24, some unique to Sars-Cov-2 and others similar to those of other coronaviruses.

At this point the researchers went looking for blood samples obtained from healthy donors they never had contact with Sars-Cov-2 the killer T cells that respond to the protein sequences that Sars-Cov-2 shares with its "cousins", thus discovering that these lymphocytes are more likely to proliferate than those that identify the unique sequences of Sars -Cov-2. According to Davis, this means that the cells that responded quickly were in "memory mode" and upon meeting an old acquaintance they activated immediately.

The next step in the research involved the analysis of samples of blood from Covid patients. People with milder symptoms had many more memory T cells on average that respond to antigenic sequences shared by several coronaviruses (including Sars-Cov-2) than people with more severe disease. "Patients with severe Covid-19 may not have been infected, at least not recently, with more delicate strains of coronavirus," comments Davis: "So they did not maintain effective memory killer T cells."

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