An antibody is a protein component of the immune system that circulates in the blood, recognizes foreign substances like bacteria and viruses, and neutralizes them. After exposure to a foreign substance, called an antigen, antibodies continue to circulate in the blood, providing protection against future exposures to that antigen.

Antibody is a part of the host cell's defense. It's made by a certain type of white blood cell that's called a B cell. The structure of the antibody consists of two light chains and two heavy chains, and at the very tip of the antibody is a hypervariable region, and this hypervariable region allows the antibody to make different types of antibodies that will respond to all of the antigens that will assault the body. An antigen is anything that is foreign to the human body. It can be a virus, it can be a bacteria, and in some cases your own body will appear as foreign. And so you can have in certain instances where your own body will make antibodies against parts that are part of you.

DEFINITIONS

• The immune system is a complex network of cells, tissues, and organs that work together to protect the body from infection.
• Antibodies are proteins that your immune system makes to help fight infection and protect you from getting sick in the future.

When you are infected with a virus or bacteria, your immune system makes antibodies specifically to fight it. Your immune system can also safely learn to make antibodies through vaccination. Once you have antibodies to a particular disease, they provide some protection from that disease. Even if you do get sick, having antibodies can protect you from getting severely ill because your body has some experience in fighting that disease. How long this protection lasts can be different for each disease, each person, or influenced by other factors. Antibodies are just one part of your immune response.

Antibodies to SARS-CoV-2, the virus that causes COVID-19, can be detected in the blood of people who have recovered from COVID-19 or people who have been vaccinated against COVID-19. Getting a vaccine is safer than getting COVID-19, and vaccination against COVID-19 is recommended for everyone 5 years of age and older. If someone has already had COVID-19, vaccination against COVID-19 increases their body’s antibody response, which improves their protection.

It is important to remember that some people with antibodies to SARS-CoV-2 may become infected after vaccination (vaccine breakthrough infection) or after recovering from a past infection (reinfected). Based on what we know right now, risk of reinfection is low for at least the first 6 months following an infection with the virus that causes COVID-19 diagnosed by a laboratory test. When someone who is fully vaccinated gets COVID-19, it is called a vaccine breakthrough infection. No vaccine is 100% effective, so some breakthrough infections are expected. The risk of infection, severe illness, hospitalization, and death are all much lower for vaccinated people compared to people who are unvaccinated. When reinfections or breakthrough infections happen, having antibodies plays an important role in helping prevent severe illness, hospitalization, and death.

For many diseases, including COVID-19, antibodies are expected to decrease or “wane” over time. After a long enough period of time, your level of antibodies can decrease below a level that provides effective protection. This level is called the “threshold of protection.” When antibodies decrease below the threshold of protection, you may become more vulnerable to severe illness. We do not yet know what the threshold of protection for antibodies is for the virus that causes COVID-19 or how long it takes these antibodies to wane. Even after antibodies wane, your immune system may have cells that remember the virus that can act quickly to protect you from severe illness if you become infected. These topics are being researched by scientists all over the world.

COVID-19 (coronavirus disease 2019) is a disease caused by a virus named SARS-CoV-2 and was discovered in December 2019 in Wuhan, China. It is very contagious and has quickly spread around the world.

COVID-19 most often causes respiratory symptoms that can feel much like a cold, a flu, or pneumonia. COVID-19 may attack more than your lungs and respiratory system. Other parts of your body may also be affected by the disease.

• Most people with COVID-19 have mild symptoms, but some people become severely ill.
• Some people including those with minor or no symptoms may suffer from post-COVID conditions — or “long COVID”.
• Older adults and people who have certain underlying medical conditions are at increased risk of severe illness from COVID-19.
• Hundreds of thousands of people have died from COVID-19 in the United States.
• Vaccines against COVID-19 are safe and effective. Vaccines teach our immune system to fight the virus that causes COVID-19.

COVID-19 is caused by a virus called SARS-CoV-2. It is part of the coronavirus family, which include common viruses that cause a variety of diseases from head or chest colds to more severe (but more rare) diseases like severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS).

Like many other respiratory viruses, coronaviruses spread quickly through droplets that you project out of your mouth or nose when you breathe, cough, sneeze, or speak.

The word corona means crown and refers to the appearance that coronaviruses get from the spike proteins sticking out of them. These spike proteins are important to the biology of this virus. The spike protein is the part of the virus that attaches to a human cell to infect it, allowing it to replicate inside of the cell and spread to other cells. Some antibodies can protect you from SARS-CoV-2 by targeting these spike proteins. Because of the importance of this specific part of the virus, scientists who sequence the virus for research constantly monitor mutations causing changes to the spike protein through a process called genomic surveillance.

As genetic changes to the virus happen over time, the SARS-CoV-2 virus begins to form genetic lineages. Just as a family has a family tree, the SARS-CoV-2 virus can be similarly mapped out. Sometimes branches of that tree have different attributes that change how fast the virus spreads, or the severity of illness it causes, or the effectiveness of treatments against it. Scientists call the viruses with these changes “variants”. They are still SARS-CoV-2, but may act differently.

Antibody or serology tests look for antibodies in your blood that fight the virus that causes COVID-19.

• Antibodies are proteins created by your immune system that help you fight off infections. They are made after you have been infected or have been vaccinated against an infection.
• Vaccination is a safe, effective way to teach your body to create antibodies.
• Antibodies can protect you from getting those infections for some period of time afterward. How long this protection lasts is different for each disease and each person.­
• Antibody tests should generally not be used to diagnose a current infection with the virus that causes COVID-19. An antibody test may not show if you have a current infection because it can take 1 to 3 weeks after the infection for your body to make antibodies.

Effect of vaccination

• COVID-19 vaccines teach your body to produce antibodies to fight infection from the virus that causes COVID-19. If you get an antibody test after receiving a vaccine, you might test positive by some (but not all) antibody tests. This depends on which type of antibody the specific test detects.
• Antibody testing is not currently recommended to determine if you are immune to COVID-19 following COVID-19 vaccination. Antibody testing should also not be used to decide if someone needs to be vaccinated. CDC’s Interim Guidelines for COVID-19 Antibody Testing provide more information on how antibody testing should be used and interpreted.

Whether you test positive or negative for COVID-19 antibodies using an antibody test, you still should take steps, including getting vaccinated, to protect yourself and others.

How to get an antibody test

Decisions about testing are made by state or localexternal icon health departments or healthcare professionals.

Antibody tests for COVID-19 are available through healthcare professionals and laboratories. Check with your healthcare professional to see if they offer antibody tests and whether you should get one.

What do your results mean?

If you test positive

• A positive antibody test result shows you may have antibodies from a previous infection or from vaccination for the virus that causes COVID-19.
• Some antibodies made for the virus that causes COVID-19 provide protection from getting infected. CDC is evaluating antibody protection and how long protection from antibodies might last. Cases of reinfection and infection after vaccination have been reported, but remain rare. But getting vaccinated, even if you have already had COVID-19, can help your body make more of these antibodies.
• You may test positive for antibodies even if you have never had symptoms of COVID-19 or have not yet received a COVID-19 vaccine. This can happen if you had an infection without symptoms, which is called an asymptomatic infection.
• Sometimes a person can test positive for SARS-CoV-2 antibodies when they do not actually have those specific antibodies. This is called a false positive.
• Talk with your healthcare professional about your test result and the type of test you took to understand what your result means. Your healthcare professional may suggest you take a second type of antibody test to see if the first test was accurate.

If you test negative

• You may not have COVID-19 antibodies. This could be because you have not had an infection with the virus that causes COVID-19 or have not received a COVID-19 vaccine.
  • Antibody testing is not currently recommended to determine if you are immune to COVID-19 following COVID-19 vaccination.
• Some antibody tests will only detect antibodies from infection, not from vaccination with the virus that causes COVID-19.
• You could have a current infection, been recently infected, or been recently vaccinated. It typically takes 1 to 3 weeks after infection or vaccination for your body to make antibodies. If you are infected, you may get sick and spread the virus before you develop antibodies.
  • Some people may take even longer to develop antibodies, and a small portion of people who are infected or vaccinated may never develop antibodies.
• Sometimes people test negative for SARS-CoV-2 antibodies when they have those specific antibodies. This is called a false negative.
• Talk with your healthcare professional about your test result and the type of test you took to understand what your result means.

If you have symptoms of COVID-19, you should get a viral test to detect a current infection, even if you were previously infected or vaccinated.

Until we know more, continue to take steps including getting vaccinated to protect yourself and others.

Serological tests detect antibodies present in the blood when the body is responding to a specific infection, like COVID-19. They detect the body’s immune response to the infection caused by the virus rather than detecting the virus itself. Experience with other viruses suggests that individuals whose blood contains antibodies associated with SARS-CoV-2 infection—provided they are recovered and not currently infected with the virus—may be able to resume work and other daily activities in society. They may also be eligible to serve as potential donors of convalescent plasma.

In the early days of an infection, when the body’s immune response is still building, antibodies may not be present in detectable levels. This limits the test’s effectiveness for diagnosing COVID-19 and is why it should not be used as the sole basis to diagnose COVID-19. Currently authorized serological tests for SARS-CoV-2 measure IgM and/or IgG antibodies. Since IgM antibodies may not develop early, or at all, in infected patients, this type of antibody test is not used to rule out SARS-CoV-2 in an individual. Since IgG antibodies generally do not develop until later, this type of antibody test, even though it is more specific to SARS-CoV-2, is not used to rule-out SARS-CoV-2 infection in an individual. We also do not know how long IgM or IgG antibodies to SARS-CoV-2 will remain present in the body after the infection has been cleared.

While antibody tests by themselves are of limited value in the immediate diagnosis of a patient where COVID-19 infection is suspected, using this type of test on many patients may help the medical community better understand how the immune response against the SARS-CoV-2 virus develops in patients over time and how many people may have been infected. While there is a lot of uncertainty with this new virus, it is also possible that, over time, broad use of antibody tests and clinical follow-up will provide the medical community with more information on whether or not and how long a person who has recovered from the virus is at lower risk of infection if they are exposed to the virus again.

Serological tests can play a critical role in the fight against COVID-19 by helping health care professionals identify individuals who may have been exposed to SARS-CoV-2 virus and may have developed an immune response. In the future, this may potentially be used to help determine, together with other clinical data, whether these individuals are less susceptible to infection.

Serological test results may also aid in determining who may qualify to donate blood that can be used to manufacture convalescent plasma as a possible treatment for those who are seriously ill from COVID-19.

A positive antibody test result can help identify someone who has had COVID-19 in the past or has been vaccinated against COVID-19. Antibody tests are not used if you have symptoms of COVID-19 or for diagnosing a current case of COVID-19. This is because it takes most people with a healthy immune system 1 to 3 weeks after getting COVID-19 to develop antibodies. A viral test is recommended to identify a current infection with the virus that causes COVID-19. More information about the role and uses of antibody testing is available.

Most people who have a positive antibody test result can continue with normal activities, including work, but they should still take steps to protect themselves and others, including getting vaccinated.

Studies show that people who have antibodies from an infection with the virus that causes COVID-19 can improve their level of protection by getting vaccinated.

A positive antibody test result alone, especially one from an infection at an unknown time or that was determined by a viral test more than 6 months ago, does not necessarily mean that you are immune to getting COVID-19. If you have had an antibody test, it is important to review your test results with your healthcare provider.

After having COVID-19, most people’s bodies develop antibodies to help fight it off. These are special molecules made by the body’s disease defense system, the immune system. A study found that people with these antibodies were less likely to get COVID-19 again.

Researchers looked at more than 3 million people who had an antibody test for SARS-CoV-2. SARS-CoV-2 is the virus that causes COVID-19.

They found that about 11% of people had SARS-CoV-2 antibodies. More than 88% had a negative test. And less than 1% of tests were inconclusive.

The scientists looked at who came down with COVID-19 after the test. They analyzed up to 30 days, 31–60 days, 61–90 days, and more than 90 days after.

About 3% to 4% of people with negative antibody tests got COVID-19 in each time period. But those who had antibodies were less likely to have COVID-19 as time went on. Only 0.3% of the people with antibodies had a positive COVID-19 test more than 90 days after. Those without antibodies were 10 times more likely to get the disease.

The findings suggest that people who have a positive result from an antibody test may be at lower risk for future infection with SARS-CoV-2.

NIH’s Dr. Lynn Penberthy, who led the research team, explains that more questions still need to be answered. “We are nevertheless encouraged by this early finding,” she says.

The science of antibodies is called “serology.” Antibody tests, also called “serology tests,” identify antibodies in blood samples. While other parts of the immune system also contribute to protection, it is easiest to test for antibodies.

As of August 2021, more than 80 antibody tests have been granted FDA emergency use authorization (EUA) to detect antibodies to SARS-CoV-2. Scientists are using these antibody tests to learn more about the level of antibodies needed to protect people from COVID-19 (threshold of protection) and how long this protection lasts. Antibody tests are not currently recommended by FDA for routine, widespread use in making individual medical decisions while this information is being gathered and evaluated. If you have questions about whether an antibody test is right for you, talk with your healthcare provider or your state or local health department.

Not all antibody tests identify the same antibodies. Some antibody tests are more or less sensitive to specific sections of the antibody protein than others. This means that different antibody tests might not have the same results, even when they are both testing for antibodies to SARS-CoV-2. Scientists use these differences in tests to help answer different research questions about how immune systems respond to the virus that causes COVID-19 and to improve our understanding of COVID-19.

As scientists learn more about the antibodies to SARS-CoV-2, we will understand a lot more about how to treat and control COVID-19.

Serological surveillance (studies that investigate antibodies in the population) provides information about how long antibody protection against COVID-19 lasts and if this protection is different among people who have antibodies from infection, compared with people who have antibodies from vaccination, or both.

We can also learn more about which groups of people might not produce as many antibodies or maintain them as long as others—for example, immunocompromised people compared with people who have healthy immune systems. This is important information for making decisions about whether or not additional vaccine doses or boosters are needed, when they would be recommended, and who would need them first.

We can also learn if antibodies to SARS-CoV-2 provide the same protection against new variants of the virus that causes COVID-19.