I can't find the study because new research into Long Covid is burying it in search results, and it's not an auotimmune disease (though it is sometimes called a "neuroimmune" disorder), but, I read a study showing that blood from patients with ME/CFS is less susceptible to flu transmission between cells in the sample than healthy control blood. This supports an old theory that ME/CFS is caused by a change in how the immune system operates that makes it more protective, at the cost of many debilitating side effects.