It’s an antidote in the immune system.

“They identified IgA (immunoglobulin A) in the respiratory tracts of several of the personnel who didn’t catch COVID-19, which could mean they had an antidote in their immune systems all this time. These antibodies are found naturally in mucous membrane secretions in the airways and gastrointestinal tract, where they protect the body by binding to viruses and other invading organisms.”

https://interestingengineering.com/immune-feature-covid19-infections