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Your blood carries the memory of every pathogen you’ve ever encountered. If you’ve been infected with the coronavirus, your body most likely remembers that, too.

Antibodies are the legacy of that encounter. Why, then, have so many people stricken by the virus discovered that they don’t seem to have antibodies? Blame the tests.

Most antibody tests offer crude yes-no answers. The tests are notorious for delivering false positives — results indicating that someone has antibodies when they do not.

But the volume of coronavirus antibodies drops sharply once the acute illness ends. Now it is increasingly clear that these tests may also produce false-negative results, missing antibodies to the coronavirus that are present at low levels.

Moreover, some tests are designed to detect a subtype of antibodies that doesn’t confer immunity and may wane even faster than the kind that can destroy the virus.

What that means is that declining antibodies, as shown by commercial tests, don’t necessarily mean declining immunity, experts said. Long-term surveys of antibodies may also underestimate the true prevalence. “We’re learning a lot about how antibodies change over time,” said Dr. Fiona Havers, a medical epidemiologist.

This is how immunity to viruses generally works: The initial encounter with a pathogen surprises the body. The resulting illness can be mild or severe. A mild illness may trigger production of only a few antibodies, and a severe one many more. The vast majority of people who become infected with the coronavirus have few to no symptoms, many experts believe, and those people may produce a milder immune response.

But even a minor infection is often enough to teach the body to recognize the intruder.

After the battle ends, balloonlike cells that live in the bone marrow pump out a small number of specialized assassins. The next time — and every time after — that the body comes across the virus, those cells can mass-produce antibodies within hours.

The mnemonic response grows stronger with every encounter. It’s one of the great miracles of the human body.

“Whatever your level is today, if you get infected, your antibody titers are going to go way up,” said Dr. Michael Mina, an immunologist at Harvard University, referring to the levels of antibodies in the blood.

A single drop of blood contains billions of antibodies, all lying in wait for their targets. Sometimes, as may be the case for antibodies to the coronavirus, there are too few to get a positive signal on a test — but that does not mean the person has no immunity to the virus.

“Even if their antibodies wane below the limits of detection of our instruments, it doesn’t mean their ‘memory’ is gone,” Mina said.

A small number of people may not produce any antibodies to the coronavirus. But they will have so-called cellular immunity, which includes T cells that learn to identify and destroy the virus. Virtually everyone infected with the coronavirus seems to develop T-cell responses, according to several recent studies.

“This means that even if the antibody titer is low, those people who are previously infected may have a good enough T-cell response that can provide protection,” said Akiko Iwasaki, an immunologist at Yale University.