At the center of the immune system's ability to be trained are T cells, which are specialized white blood cells that seek out and destroy pathogens. Multiple studies have shown that up to 50 percent of uninfected people in some places have T cells that can identify SARS-CoV-2. This may explain why some people have milder infections compared to others; however, the authors note that their findings are "highly speculative" at best, and will need more research to confirm. For one, experts are still unclear on the actual role of T cells in fighting the coronavirus.
But if these findings are proven valid, researchers say any semblance of preexisting immunity is encouraging news. In particular, the researchers found that some T cells recognized parts of the infamous “spike” protein of SARS-CoV-2, which it uses to bind to its target cells.
“We have now proven that, in some people, preexisting T-cell memory against common cold coronaviruses can cross-recognize SARS-CoV-2, down to the exact molecular structures,” explained Daniela Weiskopf, a researcher at La Jolla Institute for Immunology and a co-author of the study.
T cells give the body a head start in its battle with SARS-CoV-2
For the study, the team explored what's known as cross-reactivity between SARS-CoV-2 and human cold coronaviruses (HCoVs). Cross-reactivity happens when helper T cells developed to combat another virus react similarly to an unknown pathogen. In this case, the T cells are stragglers from previous infections with other coronaviruses – likely one of the four that cause common colds. The presence of these cross-reactive T cells, say the researchers, can help generate a faster and stronger immune response.
“You're starting with a little bit of an advantage – a head start in the arms race between the virus that wants to reproduce and the immune system wanting to eliminate it,” said co-author Alessandro Sette, in an interview with Business Insider.
The team examined blood samples collected between 2015 and 2018 from people without COVID-19 and from those who got the coronavirus and recovered. They noted that over half of the blood samples taken before the pandemic had CD4+ cells, which are T cells that activate other immune cells. CD4+ cells are capable of recognizing SARS-CoV-2 and inciting specialized immune cells to get rid of the virus and infected cells.
The researchers also found that recovered COVID-19 patients had the immune cells needed to fight the virus as well as antibodies. This meant that an average person can not only develop a good immune response against SARS-CoV-2, but he may also have immunity for some time.
Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, said that recent studies on T cells could explain why COVID-19 manifests itself differently for each patient.
“If you look at it metaphorically as an army with different levels of defense, the antibodies prevent the virus from getting in. So that’s kind of like the first line of defense. For those viruses that do escape and infect some cells, the T cells come in and kill the cells that are infected or block them,” he added.
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