The first type of antibody to appear is called immunoglobulin M or IgM, and its levels spike within a few days of infection. But IgM is a generic fighter. To target and destroy a specific virus, the body refines it into a second type of antibody, called immunoglobulin G, or IgG, that can recognize that virus.

As IgG levels rise, IgM levels drop; IgG levels peak around 28 days after the onset of infection.

There is a third type of antibody, called IgA, that is present in mucosal tissues — like the inner lining of the lung. IgA is known to be important for fighting respiratory infections such as influenza, and is likely to be central in coronavirus infection, too.

Many of the tests being developed look for levels of all three antibodies; some look for just IgM and IgG, and still others test for only one type.

What can these tests tell us? And what can’t they?

Let’s begin with what they can’t tell us. Because the antibodies come up so late, these tests are not helpful for diagnosing an early infection. “For that they are useless,” said Dr. Florian Krammer, a virologist at the Icahn School of Medicine at Mount Sinai in New York.

The tests are more effective at detecting the presence of antibody responses across large numbers of people, not just to determine who is immune but how widely the virus has spread in the population.

From 25 to 50 percent of people who become infected may never develop symptoms, and some may become only mildly ill. Others may have known they were sick, but could not get tested. Serology tests would be able to identify these people and help scientists better estimate the death rate of Covid-19, the disease caused by the virus.

“We don’t currently have good numbers for the numbers of people who are infected now, much less people who were infected before who were never tested,” said Dr. Angela Rasmussen, a virologist at Columbia University in New York. “So it’s really important from an epidemiological perspective to do these types of serology assays.”