What's all this about antibodies? Sometimes it takes a catastrophic global pandemic to make us wonder, how is my body defending itself right now?
Good question — and definitely one that we all should be able to answer. So, what are antibodies, how do they work and why are we testing for them?
Innate vs. Adaptive Immune Response
As you know, we all have an immune system — an entire government agency in the body dedicated to keeping the bad guys at bay. In order to effectively destroy the enemy — or antigens, as they're called in the immune system biz — the agency has two arms that ideally work together: The innate immune response acts very quickly, but without much of a plan, and the adaptive immune response is slower but much more specific and effective, and needs to collect data in order to work.
Our innate immune response includes a lot of initial barriers, like your skin and nose hairs, as well as specialized cellular weapons like natural killer cells, or NK cells, which cruise around the body, destroying cells infected by a virus and keeping an eye out for tumors. Our adaptive immune response, on the other hand, is the branch of the immune system that learns as much about the enemy as possible and creates specialized weapons to destroy it. Antibodies are an important tool of the adaptive immune response.
What Are Antibodies?
Your immune system makes Y-shaped proteins called antibodies to help flag foreign substances the body has deemed harmful.
"Antibodies are made by a specialized immune cell called a plasma cell, which is an activated B cell [a cell that becomes activated when its receptor recognizes an antigen, or enemy, and binds to it] that is poised to produce large amounts of protective antibodies," says Amorette Barber, professor of biology in the Department of Biological and Environmental Sciences at Virginia's Longwood University, in an email interview. "Antibodies are found in your blood and other tissues of your body and they bind to substances that appear foreign and dangerous to your body. They are very specific and can recognize and bind to many types of pathogens including viruses, bacteria, fungus and other infectious agents. Your body has billions of antibodies, each recognizing a different pathogen, and similar to a key and a lock, an antibody is incredibly specific for the pathogen it recognizes."
So, your immune system has a key on its keychain for each and every antigen it's ever fought. When an antibody binds to the pathogen it is specific for, it acts as a flag to alert the immune system that this antigen is dangerous and should be destroyed. The antibody then works with other cells and proteins of the immune system to slow the infection and clear the pathogen from the body.
And, of course, antibodies can sometimes wane and disappear, which is why vaccine boosters are necessary.
How Do Antibody Tests Work?
An antibody test shows whether a person has made antibodies against the infection that is being tested for. If a person has been exposed to a pathogen — take the COVID-19 virus, for instance — then their B cells will become activated and start making antibodies specific for that virus. And as good as antibodies are at their job, there's always a little lag time with this process: It can take a week or two for a person to start making antibodies against an infection. With particularly nasty pathogens like measles, or even COVID-19 in some cases, the disease might kill the person before their immune system has time to develop antibodies against it.
"During an antibody test, a person's blood is analyzed to see if it contains antibodies that bind to the virus," says Barber. "It does not determine if someone has an active infection, but whether they're making antibodies to the pathogen. A positive antibody test suggests that the person was exposed to the pathogen."
In essence, an antibody test indicates that a person mounted an immune response against a specific pathogen at some point, but what it can't tell is whether the person is currently infected with that pathogen.
There are different types of antibodies for a particular pathogen – some indicate recent infection and some are signs of older infection. Antibody testing means testing for all the different types. If you want to look for recent COVID-19 infection, you'd test for IgM (immunoglobulin M) antibodies. IgG (immunoglobulin G) antibodies, however, take longer to produce, so would indicate older infection. Current COVID-19 antibody testing can and does detect both types.
Antibody Tests and Immunity
A positive antibody test indicates that you have antibodies in your blood that recognize and react with that specific pathogen, which is a strong indicator that these antibodies would likely provide immunity for some amount of time. In the case of COVID-19, it's difficult to know at this point how long, or even if, these antibodies can protect you from reinfection.
"The length of protection depends on a lot of factors, including how much antibody a person makes and whether the virus mutates," says Barber.
That said, antibody tests can be used not only to determine if a person has been infected with COVID-19, but also to gain a better understanding about how widespread COVID-19 is within a community. For instance, people who were infected with COVID-19 but experienced relatively mild symptoms will still test positive on an antibody test in the same way as someone with a very acute case.
"If antibody testing is conducted on a large scale, the results can be used to more accurately estimate how many people were exposed to the virus," says Barber. "Antibody tests will also likely play a large role in getting life back to normal. People with positive antibody tests will be able to show that they developed immunity to the virus, and thus are protected from reinfection — at least for an extended amount of time."
But Barber warns antibody test results should be taken with a grain of salt, as people who test positive may still have an active virus that they can spread to others. Therefore, the use of a test that tests for the presence of the actual virus paired with an antibody test for COVID-19 will likely give a more accurate picture of whether a person is safe to return to normal life.