It seems that a few people are naturally immune to COVID-19. These people mount a T-cell response, indicating they may previously have been exposed to another virus of the coronavirus family such as SARS or MERS. T-cells are white blood cells which work in several different ways to help protect us from diseases like COVID-19. They form a part of the body’s immune system. T-cells kill infectious agents including viruses and bacteria by sometimes attacking the infection directly or the infected human cells.
T-cell response could also help to explain why some people recover relatively quickly from COVID-19. Others, with lesser T-cell response, continue to suffer chronic after-effects for months following infection. You can read more about the role of T-cells here.