A computer rendering of an antibody protein. These proteins can inhibit a virus or mark infected cells for disposal. | Getty Images/Science Photo Library

They’re just one part of an immune system that’s getting better and better at protecting you from Covid-19.

Over the past two years, the United States has seen more than 63 million Covid-19 cases, with some people infected more than once. More than 240 million people in the US have received at least one dose of a Covid-19 vaccine. More than 60 million have received three.

While Covid-19 infections are never a good thing, these numbers still add up to a glimmer of good news: A large majority of Americans now have some immunity against SARS-CoV-2, the virus that causes Covid-19. That’s a big step toward defanging the disease.

When the human body is infected by the virus or encounters a fragment of the pathogen in a vaccine, our immune systems change in subtle but important ways. Across a huge swath of the population, these changes could eventually help transform Covid-19 from a world-stopping catastrophe into a mild annoyance.

Antibodies, proteins that attach to the virus, are a critical part of the immune response and are often the center of discussions about protection from Covid-19. But they rise during infection and decline naturally over time. Fortunately, antibodies are not the whole story when it comes to the immune system.

Other, longer-lasting tools against infection are hiding inside our bones. The immune system draws on stem cells, which live in bone marrow, to produce an array of components that we don’t hear as much about. They form many kinds of white blood cells that jump into action right away when they encounter a virus for the first time, and that essentially take notes to start planning for the next infection.

It’s this immune system memory that’s key to long-term protection against Covid-19. What’s...

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