In labs all over the world lately, scientists working on COVID-19 have stumbled on an intriguing sort of finding again and again. They’ve found that blood samples from healthy people who were never exposed to the SARS-CoV-2 coronavirus contain reactive immune cells and targeted antibodies that could, perhaps, help stave off COVID-19. 

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These people may—it is still just a hypothesis—possess some degree of pre-existing immunity. If correct, it’s even possible that this immunity has saved thousands from the worst manifestations of this terrible disease.

Some of the first hints of pre-existing immunity came via T cells, the white blood cells that destroy infected cells in the body or help other parts of the immune system target an invading pathogen. In one study originally published as a preprint on medRxiv April 22, a group of scientists in Germany reported an intriguing result.