Trump's doctor says the president already has antibodies. That might not mean he is cured with long-term immunity — it could just be his medication.

• Trump's doctor announced that lab tests showed the president had "detectable levels" of coronavirus antibodies on Wednesday, less than a week after Trump's diagnosis.
• Your body creates antibodies to help fight off the coronavirus, but research shows these protective proteins take between one and three weeks to develop after you get infected.
• While hospitalized with COVID-19, Trump took a high dose of Regeneron's experimental antibody cocktail.
• Experts suggest that the tests picked up antibodies from the cocktail, not antibodies made by Trump's immune system. Regeneron said that is likely the case.
• If these are self-made antibodies, one doctor said, it suggests Trump may have been infected earlier than previously thought.

White House physician Sean Conley reported that President Trump has "detectable levels" of coronavirus antibodies.

The president, who was diagnosed with COVID-19 Thursday, recently returned to the White House from Walter Reed Military Medical Center. According to Conley, Trump's blood tested positive for Ig antibodies — a type of protective protein that helps us fight off infection — on Monday.

That announcement left several experts scratching their heads. Typically, it takes the immune system between one and three weeks to make enough antibodies that can be detected in a lab. But a peek at Trump's treatment at Walter Reed may explain why he has antibodies just six days after testing positive for COVID-19.

In his report, Conley failed to mention that Trump recently took a single, 8 mg dose of an experimental antibody cocktail called REGN-COV2 from drug company Regeneron that's been shown to improve symptoms in non-hospitalized patients.

"This is a really wild thing," Peter Chin-Hong, an infectious disease expert at the University of California, San Francisco, told Business Insider. "Most likely, it's a false positive because of the antibody cocktail he got on Friday."

The test 'likely' detected antibodies from the cocktail

A spokeswoman from Regeneron suggests Chin-Hong's assessment is on point.

Given the timing of the test, and the fact that Trump received an 8 mg dose — the higher of two doses from REGN-COV2's clinical trial by more than a factor of three (the other dose was 2.4 mg) — "it's likely" that the test is detecting antibodies from the Regeneron cocktail, company spokeswoman Hala Mirza, said in a statement Wednesday.

Most tests that look for immunoglobulin G (IgG), the most common antibody found in our blood, cannot distinguish between endogenous antibodies, meaning the ones the patient's immune system made, and antibodies "delivered by our therapy," Mirza added.

Antibodies show up between one and 3 weeks after a person gets infected, meaning Trump could still be infectious

Antibodies help the body identify invading pathogens, then mark them for destruction by aggressive white blood cells.

Some people develop them within the first week of getting sick, according to the CDC. But in other COVID-19 infections, Ig antibodies arise "within two to three weeks after illness onset."

Research suggests you should wait at least two weeks after the onset of COVID-19 symptoms before taking an antibody test.

"In medicine, we don't check for antibodies much more than two weeks after infection because they take some some time to develop," Chin-Hong said.

According to Conley, Trump's medical team detected his antibodies just four days after Trump's positive diagnosis on Thursday. Bloomberg News reported that a few aides think Trump might have been exhibiting symptoms a day earlier.

That means that Trump could still have an active coronavirus infection, since the virus's incubation period can be up to two weeks long.

"He should be wearing a mask," Chin-Hong said, adding: "You can have antibody detection and still be infectious. They're not mutually exclusive."

If the antibodies aren't from the drug cocktail, this could mean Trump's been sick for longer than a week

If Trump's antibodies are, indeed, endogenous, that could suggest the president has been sick for longer than his diagnosis timeline indicates.

"It is possible that he was infected for longer than reported as there has not been complete clarity on when he developed symptoms and when he last tested negative," Monica Gandhi, a professor of medicine at the University of California, San Francisco, told Business Insider via email.

That could be somewhat good news for those concerned about President Trump's health.

Typically, patients with severe COVID-19 take a turn for the worse between the 7- to 10-day window, which is likely why Trump's doctors said they were still "on guard" Monday.

But according to Chin-Hong, that downturn may have already happened for Trump when he went to the hospital over the weekend.

"If you go back in time, he could've had it two weeks ago from Monday," he said, adding, "that suggests this might now be the second phase of his disease, rather than the first."

Gandhi agreed: "The development of IgG antibodies is usually associated with a decrease in symptoms," she said, adding, "so, yes, this is a good sign for him in terms of hope for not having severe symptoms."

This doesn't necessarily mean Trump is immune

Even if Trump's antibodies are the real deal, and not from the Regeneron cocktail, that doesn't necessarily mean he has long-term coronavirus immunity.

With some diseases, like measles and hepatitis A, infection is a one-and-done deal. Once you get sick and recover, you're immune for life.

"For human coronaviruses, that's not the case," Florian Krammer, a vaccinologist at the Icahn School of Medicine at Mount Sinai, previously told Business Insider. "You can get repeatedly infected once your immunity goes down."

What's more, it's still unclear what level of antibodies a person needs to be considered immune.

With many better-studied viruses like measles, Krammer said, "you know how many antibodies you need to be protected."

But we don't have a specific number for the coronavirus.

Reports of coronavirus reinfections lend further credence to the idea that our immunity may be fleeting.

"We do not know how long immunity lasts or if patients can have full immunity from natural infection," Gandhi said.

That's why — even in light of this antibody news — Trump can't say he's now immune, she added.

—Donald J. Trump (@realDonaldTrump) October 5, 2020

The president alluded to his potential protection from the virus in a Twitter video Monday, saying: "Now I'm better and maybe I'm immune."