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The presence of antibodies that attack the immune system may be the reason why men are more likely to die of a serious case of COVID-19 than women, new research has found.

The destructive antibodies — called autoantibodies — were found in 10 percent of nearly 1,000 COVID-19 patients ranging from 25 to 87 years old who had developed life-threatening pneumonia, according to a study published last month in Science and reported by NBC News.

Of those 101 patients with autoantibodies, 94 percent were men.

Autoantibodies attack immune system proteins called interferon — which respond to the entry of a virus in the body — and are found in patients with autoimmune diseases, like lupus and rheumatoid arthritis.

In the study, roughly 12.5 percent of male COVID-19 patients with pneumonia had autoantibodies against interferon versus 2.6 percent of women.

Autoantibodies, meanwhile, were missing in 663 COVID-19 patients who had mild or asymptomatic cases — and just four of 1,227 healthy patients had them.

“This is one of the most important things we’ve learned about the immune system since the start of the pandemic,” Dr. Eric Topol, executive vice president for research at Scripps Research in San Diego, told NBC News.

“This is a breakthrough finding,” added Topol, who wasn’t involved in the study.

Sabra Klein, a professor of molecular microbiology and immunology at the Johns Hopkins Bloomberg School of Public Health, said the results were unexpected — autoimmune disease is more common in women than men.

“I’ve been studying sex differences in viral infections for 22 years, and I don’t think anybody who studies autoantibodies thought this would be a risk factor for COVID-19,” Klein told NBC.

She said the finding could explain why men are more likely than women to die of a serious case of COVID-19.

“You see significantly more men dying in their 30s, not just in their 80s,” she said.

Paul Bastard, one of the authors of the study, said testing COVID-19 patients for autoantibodies against interferon could help determine which are more likely to come down with serious cases.

Testing, which takes about two days, is available now in Paris hospitals with a request from a doctor, Bastard said.

“I think we should give the test to everyone who is admitted,” he said. Otherwise, “we wouldn’t know who is at risk for a severe form of the disease.”