Unpredictability of COVID-19 severity may come down to antibody response, study shows

More than a year into the COVID-19 pandemic, experts still can’t determine which cases will be mild and which will result in severe illness.

But thanks to a new study published by Nature Medicine, researchers might be gaining a better understanding of why an infected person would become seriously ill.

As Dr. Francis Collins wrote in a blog post for the National Institute of Health, it comes down to the type of antibody response the person produces.

The type of antibody response produced by an adult with mild COVID-19 is remarkably similar to the type produced by kids with mild infections, the study showed. The difference comes into play when examining severe cases in adults and complications in minors.

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A small percentage of infected kids develop a delayed complication called multisystem inflammatory syndrome in children (MIS-C). While examining their plasma, researchers detected high levels of long-lasting immunoglobulin G (IgG) antibodies.

IgG would typically help control an acute infection, but the study suggests those antibodies may activate scavenging immune cells, called macrophages, to drive inflammation and more severe illness.

In adults with severe illness, researchers discovered the presence of a different type of antibody called "immunoglobulin A" (IgA).

This transmission electron microscope image shows SARS-CoV-2-also known as 2019-nCoV, the virus that causes COVID-19-isolated from a patient in the U.S. (Photo by: IMAGE POINT FR - LPN/BSIP/Universal Images Group via Getty Images)

IgA interacted with immune cells called neutrophils, which led to the release of cytokines. The presence of too many cytokines is known as a "cytokine storm," which is a severe symptom of COVID-19 associated with respiratory distress syndrome, multiple organ failure and other life-threatening complications.

Collins acknowledged more research is needed to understand the immune response over time in both kids and adults. But he hopes these findings and others will help put researchers on the right path to discover better ways to help protect people of all ages from the most severe complications of the disease.

"To understand how a single virus can cause such different outcomes, studies like this one help to tease out their underlying immune mechanisms," Collins wrote.

This story was reported from Atlanta.