Dangerously ill coronavirus patients are making "startling recoveries" in spite of being at "death's door" after being given drugs that dial down the immune system, experts have said. Trials are taking place of several drugs that prevent a part of the immune system called the complement system from becoming over-activated. The drug furthest along in trials, ravulizumab, is already used to treat rare blood diseases and is being tested at hospitals in Cambridge, London, Birmingham and Leeds. The drugs are known as "anti-C5" drugs because they prevent a molecule called C5 from triggering the complement-system response. Speaking at a coronavirus briefing on Thursday, Professor Paul Morgan, the director of the Systems Immunity Research Institute at Cardiff University, said the drugs were providing a lifeline for patients who were near death. He said: "Switching off C5 can have a big effect. We and others have used anti-C5 blocking agents in small scales on very severe Covid patients with very promising results. "These were people who had reached the stage where there was no further therapy for them; they were on ventilators, and really at death's door ... [some] have made startling recoveries. "Of course these are small numbers, but these drugs are now in large scale clinical trials and we want to see the outcomes of those in the too distant future." The complement system helps clear away harmful cells and triggers the production of immune cells known as cytokines which can cause inflammation. However, when in overdrive it begins attacking the body itself and is thought to play a role in many autoimmune diseases, including asthma, arthritis, multiple sclerosis and inflammatory bowel disease. It is also the response that causes sepsis. In the early stages of the disease, Covid-19 is believed to switch off the body's ability to make the anti-viral proteins called interferons. It is the reason patients do not feel unwell even when they have a lot of virus in the body. Although anti-viral drugs such as remdesivir have not proved as successful as hoped in trials, it is possible they may work earlier in the illness to stop the immune system from overloading. Paul Lehner, professor of immunology and medicine at the University of Cambridge, said it was crucial to try and treat the disease before the dangerous immune storm had happened. "We have to get better at asymptotic screening and we need to treat those at risk early," he said. "We are identifying now, I think, good and better inhaled antiviral agents. We've got to learn how to treat early to avoid the severe stage disease. Inhaled interferons or remdesivir may be effective in the early stage." Professor Sir Stephen O'Rahilly, the director of the MRC Metabolic Diseases Unit at the University of Cambridge, also said people could help themselves by losing weight. Sir Stephen, who caught coronavirus in the spring, believes he only survived because he had lost 13 pounds in the preceding months and said: "Even a small amount of weight loss can be beneficial. Walk a mile, lose a pound. Even a modest degree of calorific restriction in a matter of days can start to shift fat in the organs even before body weight reduces. "We might be able to accelerate this with diabetes drugs, using them in people who don't have diabetes, to improve insulin sensitivity."