The term “neutrophils” is little known to the general public, but in fact this family of white blood cells may play a key role in the body’s reaction to Covid-19. “Circulating neutrophils in the blood are the first cells to be recruited in the event of viral intrusion into a tissue. These cells from the immune system then leave the bloodstream and enter the infected area – the lung, in the case of a Covid-19 infection,” explains Mireille Laforge, an immunologist at the environnemental Toxicity, Therapeutic Targets, cellular Signaling and Biomarkers laboratory (T3S), who with other scientists from the CNRS, INSERM and Université de Paris has authored an article on this subject in Nature Reviews Immunology.

Once in the infected tissue, the neutrophils are activated and release free radicals, or reactive oxygen species (ROS) that destroy the infective agent. The problem is that although ROS are highly effective weapons against intruders, these toxic derivatives may also escape control by the body and turn back on it, attacking its healthy cells. “Under normal circumstances, the body benefits from antioxidant defences that rapidly neutralise ROS. But these decline with age or in the case of pathologies such as diabetes, obesity and other chronic conditions, thus leading to oxidative stress,” she adds.

This means that an excessive activation of neutrophils and a particularly abundant production of their fatal weapon, ROS, may be involved in severe forms of Sars CoV-2. “This theory, verified by several international studies, has been supported by the presence in Covid-19 patients of an abnormally high proportion of neutrophils in the blood, all the more so in the case of an acute form of the disease.”