There is even a small chance that pre-existing T cells could raise the risk for serious symptoms of Covid-19, although experts consider this possibility unlikely. T cells that are primed to recognize common-cold coronaviruses might marshal only a lackluster response to the current coronavirus, potentially sapping resources from other populations of immune cells that have a better shot at defeating the new invader. “Now you have your immune system distracted,” Dr. Iyer said.
T cells are also expert orchestrators. Depending on the signals they send out, they can synchronize cells and molecules from disparate parts of the immune system into a tag-teamed attack, or quell these assaults to return the body to baseline. If it turns out that cross-reactive T cells tend toward quieting the response, they could suppress a person’s immune defense before it has a chance to kick into gear, Dr. August said.
Then again, many types of T cells exist, and all operate as part of a complex immune system. “It’s almost like some people are trying to say this is ‘good’ or ‘bad,’” Dr. Su said. “It’s probably more nuanced than that.”
Teasing it all apart will not be easy. Unlike antibodies, which are inanimate proteins that often circulate in the blood, T cells are living cells that often hole up in hard-to-reach tissues. That makes them much more difficult to extract, maintain and analyze, Dr. Pepper said.
Researchers could learn more by testing whether cross-reactive T cells are more abundant in patients who have had mild or serious cases of Covid-19, although such studies cannot prove cause and effect. A more laborious effort might involve measuring cross-reactive T cell levels in large groups of healthy people, then waiting to see if they became infected or sick from the current coronavirus, Dr. Sette said.
Strong evidence could also come from an animal model, like the rhesus macaques that Dr. Iyer studies in her lab. Researchers could dose primates with common-cold coronaviruses, and then see how their immune responses stack up against the new coronavirus.
Less than a year into this pandemic, plenty of questions remain unanswered, Dr. Pepper said. Immunologists cannot fully forecast how the human immune system will respond to this new virus; even with science at its speediest, that interaction must be studied in real time.
It’s a frustrating reality, Dr. Pepper said: “Until we see it in real life, we just don’t know.”
