It seems that a few people are naturally immune to COVID-19 because they 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 and 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.
A T-cell response could also help to explain why some people recover relatively quickly from COVID-19, but others continue to suffer chronic after-effects for months following infection. You can read more about the role of T-cells here.