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Coronavirus: T-cell memory may ensure immunity after antibodies are gone

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In order to verify whether an individual is protected against the coronavirus – whether after recovery or after getting vaccinated – the most common tool used is to assess the presence of antibodies in their blood and to assess their level. However, this approach does not take into consideration another important part of the immune system, the T cell memory.

Albeit more challenging to evaluate, T cell memory represents a fundamental factor in the immunity against the virus and may be able to ensure protection after the antibodies are gone, experts said.

“We have two immune systems in our body, one that is called innate immune system, and one that is called adaptive immune system,” Prof. Cyrille Cohen, head of the immunotherapy laboratory at Bar-Ilan University, said. “The adaptive immune system is composed of two types of cells, B cells that can make antibodies and T cells that can kill cells in our body that are infected by viruses or that are sick with cancer.”

Cohen explained that when the body first encounters a pathogen, it takes some time for it to understand how to make the best antibodies and how to make the best T cells to fight it, around a week.

“At this point, the body produces a lot of cells that make antibodies and a lot of T cells,” the professor said. “When the threat is over, most of those cells die, but a tiny percentage of them remain and this is what we call the memory T cells, or memory B cells.”

Already trained to fight the specific disease, these cells reactivate if needed even after a long time, allowing the body to respond to the threat in one or two days instead of a week.

“Maintaining a high level of antibodies would be impractical for the body so the immune system has this other beautiful mechanism,” Hebrew University Prof. Michal Linial, director of the Center for Computational Biology, said.

However, testing the presence and effectiveness of these memory cells is much harder, requiring procedures and equipment that can be found only in specialized laboratories.

“Part of the reason is that their quantity is very limited, part of it is that they need to be extracted from the bone marrow, which is a pretty painful procedure,” Linial said.

The experts noted that it is too early to have a complete picture of the situation regarding immunity against COVID.

Cohen however said that the first data are promising.

“For example, a recent study showed that there was no significant difference between T cell memory of asymptomatic patients and symptomatic patients, and therefore the severity of the disease does not appear to impact the number of T cells,” he remarked.

“Moreover, we have seen that this memory cells can last quite long, a few months after the infection,” he added.

In addition, there are indications that T cell memory developed due to infection by another type of coronavirus, which is a family of several viruses, might also be effective against COVID-19.

According to Cohen, it is going to take some time, and especially some real world data, to have a clearer picture of the situation.

“People have to remember that we are dealing with a new virus and a new type of vaccine,” he said. “However, I’m very optimistic.”

“It is really early, but my estimation is that those who appear to have a very high level of antibodies will likely have also a high level of T cell memory, but those who have a low level of antibodies won’t probably have that,” Linial remarked.

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