For almost three years, scientists have raced to understand the immune responses in patients who develop severe COVID-19, with an enormous effort aimed at defining where healthy immunity ends and destructive immunity begins.

In the early days of the COVID-19 pandemic, much attention focused on reports of harmful inflammation and so-called cytokine storms — dangerous immune overreactions that can lead to tissue damage and death — in patients with severe COVID-19.

It wasn’t long before researchers began to identify antibodies that target the patient’s own body rather than attacking SARS-CoV-2.

Those studies revealed that patients with severe COVID-19 share some of the key traits of chronic autoimmune diseases — diseases in which the patient’s immune systems chronically attack their own tissues.