T cells are a kind of immune cell, whose main purpose is to identify and kill invading pathogens or infected cells. It does this using proteins on its surface, which can bind to proteins on the surface of these imposters. Each T cell is highly specific – there are trillions of possible versions of these surface proteins, which can each recognise a different target. Because T cells can hang around in the blood for years after an infection, they also contribute to the immune system's "long-term memory" and allow it to mount a faster and more effective response when it's exposed to an old foe.
Several studies have shown that people infected with Covid-19 tend to have T cells that can target the virus, regardless of whether they have experienced symptoms. So far, so normal. But scientists have also recently discovered that some people can test negative for antibodies against Covid-19 and positive for T cells that can identify the virus. This has led to suspicions that some level of immunity against the disease might be twice as common as was previously thought.
Most bizarrely of all, when researchers tested blood samples taken years before the pandemic started, they found T cells which were specifically tailored to detect proteins on the surface of Covid-19. This suggests that some people already had a pre-existing degree of resistance against the virus before it ever infected a human. And it appears to be surprisingly prevalent: 40-60% of unexposed individuals had these cells.