T cells are a species of immune cells that have the purpose of identifying and eliminating invading pathogens or infected cells, using proteins on their surface that can in turn adhere to that of imposters.
Each of these cells is very specific and there are trillions of possible variations that can recognize a different target. Studies have shown that people infected with COVID-19 often have T cells that can attack the virus even if the person has had symptoms.
Scientists also found that some people test negative for COVID-19 antibodies and positive for T cells, which identify the virus. The funny thing is that, according to blood tests taken before the pandemic began, they found T cells designed to detect proteins on the surface of covid-19. That is, some people already had resistance to this virus long before it a human will be infected.