While the latest research suggests that antibodies against Covid-19 could be lost in just three months, a new hope has appeared on the horizon: the enigmatic T cell.
The clues have been mounting for a while. First, scientists discovered patients who had recovered from infection with Covid-19, but mysteriously didn’t have any antibodies against it. Next it emerged that this might be the case for a significant number of people. Then came the finding that many of those who do develop antibodies seem to lose them again after just a few months.
In short, though antibodies have proved invaluable for tracking the spread of the pandemic, they might not have the leading role in immunity that we once thought. If we are going to acquire long-term protection, it looks increasingly like it might have to come from somewhere else.
But while the world has been preoccupied with antibodies, researchers have started to realise that there might be another form of immunity – one which, in some cases, has been lurking undetected in the body for years.
An enigmatic type of white blood cell is gaining prominence. And though it hasn’t previously featured heavily in the public consciousness, it may well prove to be crucial in our fight against Covid-19.
This could be the T cell’s big moment.
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.
The central role of T cells could also help to explain some of the quirks that have so far eluded understanding – from the dramatic escalation in risk that people face from the virus as they get older, to the mysterious discovery that it can destroy the spleen.