What gives some people immunity from the novel coronavirus, the virus that causes COVID-19? Some scientists are studying whether it is the presence of antibodies. Others are studying a part of the human immune system, known as T cells, which can protect people from viruses. Recent studies show that some recovered patients who tested negative for coronavirus antibodies developed T cells in their immune systems. The reaction is believed to be the result of their COVID-19 infection. The studies are small and have yet to be confirmed by outside experts. However, some scientists now say that people who show few signs of sickness from the new coronavirus, or show no signs at all, may be actually killing off the infection with their T cells.