Common colds may have left some people's immune systems with a leg up in fighting the new coronavirus

• In our immune systems, T cells identify infected cells, kill them, and tell B cells how to create new antibodies.
• Some research has found that people who've never had the new coronavirus still have T cells that can identify and react to it.
• That may be because those people were previously exposed to other coronaviruses — those that cause common colds — and their T cells remember.
• That preexisting immunological memory may help explain why some people's COVID-19 cases remain so mild.

A cold you got years ago may prove helpful if your body has to fight the new coronavirus.

According to a study published Tuesday, some people who've never been exposed to the new coronavirus may nonetheless have T cells that react to it. Scientists think that's because those cells previously learned how to identify and fight coronaviruses that cause common colds.

A type of white blood cell, T cells are a crucial part of the body's defense against a virus: They identify and destroy infected cells while also informing B cells about how to craft new antibodies. When you're infected, your immune system generates both antibodies and these white blood cells.

Antibody levels can drop in the months following an infection, but memory T cells stick around for years and can help mount another attack should the same virus ever return.

Recent research suggests that T cells that remember how to fight other coronaviruses may give people an immunological head start against the new coronavirus.

"This could help explain why some people show milder symptoms of disease while others get severely sick," Alessandro Sette, a coauthor of the new study, said in a press release. He cautioned, though that it's too soon to tell whether that preexisting immunological memory affects COVID-19 patients' outcomes.

Some T cells recognize the new coronavirus without having seen it before

Sette's team analyzed blood samples collected between 2015 and 2018 from 25 people who, of course, had never had COVID-19. They found that those unexposed individuals had memory T cells that could recognize both the new coronavirus and the four types of common cold coronaviruses.

Those findings built on research Sette published in May, in which he which described 10 people who had never been exposed to the new coronavirus yet had helper 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.

"Preexisting immune reactivity exists to some degree in the general population," Sette wrote in the analysis.

Two other recent studies offer even more evidence for this conclusion.

The first, published last month, found that among 68 healthy Germans who'd never had COVID-19, more than one-third had T cells that reacted to the virus. The second, 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 recognize the new coronavirus.

The Nature study also examined 23 people who'd survived SARS — which is a coronavirus, too — and found that they still had SARS-specific memory T cells 17 years after getting sick. Those same T cells could recognize the new coronavirus as well.

People with cross-reactive T cells might mount a faster immune response

The likeliest 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.

That can give the immune system a leg up.

"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," Sette previously told Business Insider.

In the absence cross-reactive T cells, your body has to mount its defense from scratch — which could impact how expediently your immune system can respond to the invading virus. Varying levels of cross-reactivity might therefore "translate to different degrees of protection," Sette said.

"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," he added.