The world is fighting the novel coronavirus (Covid-19) for the eighth month of 2020. Accelerated research about Covid-19 and development of potential vaccines are happening. A latest, promising information is that T cells, a type of white blood cell and a key fighter in human bodies against a virus, may give people an immunological head start against Covid-19. This could help explain why some people show milder symptoms of disease while others get severely sick, according to Alessandro Sette, a co-author of a new study. He cautioned, though that it’s too soon to tell whether that pre-existing immunological memory affects Covid-19 patients’ outcomes.
According to the study, some people who have never been exposed to Covid-19 may have T cells that react to it as they previously learned how to identify and fight coronaviruses that cause common colds. Some other recent studies have also revealed that 20% to 50% of people in some places might harbour T cells. These memory T cells, are most likely leftovers from past fights with other, related coronaviruses, including four that frequently cause common colds. When we are infected, our immune system generates both antibodies and T cells. Though antibody levels can drop in the months following an infection, the T cells can hang around for years and help mount another attack should the same virus ever return.
Sette’s team analysed blood samples collected between 2015 and 2018 from 25 people who, of course, had never had Covid-19. They had memory T cells that could recognise both the new coronavirus and the four types of common cold coronaviruses. These findings built on research Sette published in May, in which he described 10 people who had never been exposed to the new coronavirus yet had T cells capable of identifying and responding to it. He also did a larger analysis looking at data from cohorts in the US, Netherlands, Germany, Singapore, and the UK, and concluded that white blood cells from 20% to 50% of unexposed people significantly react to the new coronavirus.
Another study published last month, found that among 68 healthy Germans who had never had Covid-19, more than one-third had T cells that reacted to the virus. A third study, published in the journal Nature, found that more than half of a group of 37 healthy people who had never gotten Covid-19 had memory T cells that could recognise the new coronavirus. The Nature study also examined 23 people who survived Severe acute respiratory syndrome (Sars) — which is also a coronavirus, — and found that they still had Sars-specific memory T cells 17 years after getting sick. Those same T cells could recognise the new coronavirus as well.
The most likely explanation for these observations is a phenomenon called cross-reactivity: when T cells developed in response to one virus react to a similar, but previously unknown, pathogen. This can give a shot in the arm for the immune system. Having a strong T cell response, or a better T cell response may give you the opportunity to mount a much quicker and stronger response, Sette said. Marion Pepper, an immunologist at the University of Washington who is studying the immune responses of Covid-19 patients, was of the view that “If you have a population of T cells that are armed and ready to protect you, you could control the infection better than someone who doesn’t have those cross-reactive cells.” In theory, cross-reactive T cells can “protect almost like a vaccine,” said Smita Iyer, an immunologist at the University of California, Davis, who is studying immune responses to the new coronavirus in primates. The world hopes for the best outcome as the war against Covid-19 continues.
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