Some people’s immune systems may have a head start in fighting the coronavirus, recent research suggested. A study published last month in the journal Cell showed that some people who have never been exposed to the coronavirus have helper T cells that are capable of recognizing and responding to it. The likeliest explanation for the surprising finding, according to the researchers, is a phenomenon called cross-reactivity: when helper T cells developed in response to another virus react to a similar but previously unknown pathogen. In this case, those T cells may be left over from people’s previous exposure to a different coronavirus — likely one of the four that cause common colds.

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,” explained Alessandro Sette, one of the study’s coauthors. He added that cross-reactive helper T cells could “help generate a faster, stronger immune response.”  For its study, Sette’s team examined the immune systems of 20 people who got the coronavirus and recovered, as well as blood samples from 20 people that had been collected between 2015 and 2018 (meaning there was no chance those people had been exposed to the new coronavirus).

Among the 20 confirmed COVID-19 patients, the researchers found, every person had both the white blood cells specifically engineered to fight the virus and the resulting antibodies.”The data are suggestive that the average person makes a good immune response and may have immunity for some time,” commented Shane Crotty, another coauthor of the study. He added that this finding probably meant that “the many vaccines people are trying to make should be able to replicate natural immunity.” Among the 20 people whose blood samples were taken before the pandemic, 50% had a type of white blood cell called CD4+ — T cells that help the immune system create antibodies — that the researchers found to be capable of recognizing the new coronavirus and prompting the immune system to fight back right away.