As COVID-19 cases fill the hospitals, among the sickest and most likely to die are those whose bodies react in a signature, catastrophic way. Immune cells flood and attack the lungs they should be protecting. Blood vessels leak; the blood itself clots. Blood pressure plummets and organs start to fail.
Such cases, doctors and scientists increasingly believe, are due to an immune system gone overboard — so that it harms instead of helps.
Normally, when the human body encounters a germ, the immune system attacks the invader and then stands down. But sometimes, that orderly army of cells wielding molecular weapons gets out of control, morphing from obedient soldiers into an unruly, torch- and pitchfork-bearing mob. Though there are tests and treatments that could help to identify and tamp down this insurrection, it’s too early to be sure of the best course of therapy for those who are suffering a storm due to COVID-19.
Variants on this hyperactive immune reaction occur in an array of conditions, triggered by infection, faulty genes or autoimmune disorders in which the body thinks its own tissues are invaders (see box). All fall under the umbrella term “cytokine storm,” named because substances called cytokines rampage through the bloodstream. These small proteins — there are dozens — are the immune army’s messengers, transiting between cells with a variety of effects: Some ask for more immune activity; some request less.
