Months after recovery from mild COVID-19, when antibody levels in the blood have declined, immune cells in bone marrow remain ready to pump out new antibodies against the coronavirus, researchers reported on Monday in Nature. Upon infection, short-lived immune cells are generated quickly to secrete an early wave of protective antibodies. As the immune cells die out, antibody levels decline. But a pool of these immune cells, called long-lived plasma cells, is held in reserve after infection. Most of them migrate to the bone marrow, explained coauthor Ali Ellebedy of Washington University School of Medicine in St. Louis.