What is an antibody? Your immune system response to coronavirus explained

Here's how antibodies help protect your body from harmful invaders.
A computer model shows the immunoglobulin G antibody molecule. Each Y-shaped molecule has two arms (top) that can bind to bacterial or viral proteins. In doing this, they mark the enemy for destruction by white blood cells.
A computer model shows the immunoglobulin G antibody molecule. Each Y-shaped molecule has two arms (top) that can bind to bacterial or viral proteins. In doing this, they mark the enemy for destruction by white blood cells.Getty Images

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/ Source: TODAY
By A. Pawlowski

When pathogens like the new coronavirus attack, the body creates antibodies to neutralize the invader — an important part of the immune response.

It’s why some researchers are trying out convalescent plasma therapy as a way to help people sick with COVID-19.

The approach involves collecting antibodies from the blood of patients who have recovered and injecting the serum into those who are still very ill in the hope they’ll be able to get better faster.

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What exactly are antibodies and how do they work? TODAY asked Dr. Daniel Kuritzkes, chief of the division of infectious diseases at Brigham and Women's Hospital in Boston and professor of medicine at Harvard Medical School.

What is an antibody?

In basic terms, it’s a protein produced by the immune system in response to invading organisms such as bacteria and viruses.

“Antibodies are able to specifically recognize foreign invaders in the body, coat them by binding to them and then allow the immune cells of the body to clear them from the blood or mucous membranes, or sites like the lung,” Kuritzkes said.

Each antibody can bind to only one specific enemy substance. Some destroy it directly; others make it easier for white blood cells to destroy the pathogen, according to the National Cancer Institute.

This computer illustration shows antibodies (depicted in pink) attacking a virus particle (depicted in blue).Getty Images

Researchers are most interested in neutralizing antibodies, which actually block the infection of cells. Only a small subset of antibodies are capable of doing this.

Antibodies are also part of the immune response doctors try to generate when they give vaccinations.

How fast are antibodies produced?

Typically, it takes a couple of weeks after an infection starts for antibodies to first become detectable in the blood. With the new coronavirus, preliminary data suggests they can be detected within seven to 10 days after infection, Kuritzkes noted.

In general, the amount of antibody that is present declines over time after an infection has been cleared.

Sometimes, doctors can still measure antibodies after many years — as is the case with measles; and sometimes the antibodies go away more quickly, he added.

“But one of the really clever things about the immune system is that the cells that make this antibody then persist at some low level and form a population of memory cells that are there and able to respond much more briskly if the same pathogen or invader is encountered again on another occasion,” Kuritzkes said.

“It would be that kind of memory that may be able to provide protective immunity against re-infection.”

It’s not known yet how long the novel coronavirus antibodies stay in the blood since the pathogen is so new.

“Different infections have different patterns of persistence of the antibody and we just don’t know at this point how long to expect these antibodies to hang around,” Kuritzkes noted.

What are the different types of antibodies?

The immune response includes initial and secondary antibodies:

  • Immunoglobulin M, or IgM, is the initial antibody made to fight an invader
  • Immunoglobulin A, or IgA, is typically found in saliva and other secretions
  • Immunoglobulin G, or IgG, is the main type of antibody found in all body fluids

After initially making IgM, the cells switch over to making IgG, which is the longer-lived kind of antibody, Kuritzkes said.

The three types are often measured together, so the presence of IgM means a person is currently infected or very recently became infected. Having IgG means the infection could have happened a month or two ago.

How are antibodies detected?

In general, a drop of blood from a finger prick, a routine blood sample or even saliva can be used. Saliva tests are not yet available for the new coronavirus antibodies, but researchers are working on them, Kuritzkes said.

The presence of antibodies can be key to diagnosing certain kinds of infections where it's hard to detect the actual invading organism.

Can antibodies be transferred to another person and provide protection?

That’s been a big topic of interest and debate with the new coronavirus.

Some studies suggest there may be a benefit for COVID-19 patients, but they’re small and hard to interpret, Kuritzkes noted.

“Convalescent serum has been used in other viral infections with spotty results and it’s never really become a mainstay of therapy,” he said.