Antigens and antibodies play vital but distinct roles in illness and disease. One tries to wreak havoc on our health while the other attempts to guard it.
Read on to find out which role antigens and antibodies play and how they accomplish these.
What is an antigen?
Antigens (or immunogens) are substances that prompt an immune response. They rouse lymphocytes or your body’s illness-fighting white blood cells to make antibodies against them.
Antigens identified and suppressed by antibodies tend to be proteins such as receptors found on cancer cells, but can also be:
- • Carbohydrates (polysaccharides)
• Lipids
• Hormones
• Chemical compounds
• Nucleic acids
Heteroantigens are foreign to your body and include substances made by or found within:
- • Viruses
• Microorganisms such as bacteria and protozoa
• Blood and red blood cells from other people
• Snake venom
• Allergens such as pollen
• Certain proteins in foods
Autoantigens or self-antigens comprise the other main type and are made within your body. When you have an autoimmune disease, your body makes autoantibodies which mount an immune response that target and react against your own cells.
What is an antibody?
Antibodies (or immunoglobulins or Ig) are Y-shaped proteins made by your immune system’s B-lymphocytes (or B-cells). They bind to antigens and tag them for attack.
They then neutralize them by blocking entrance into your healthy cells, killing them, or flagging other immune system proteins to gird and ravage them in a process known as phagocytosis.
The main types of antibodies include:
- • IgG are the most prolific type in your plasma. They detoxify harmful substances and provide long-term protection.
• IgM are the first to be produced by B cells in response to antigens. They promote phagocytic ingestion of cells.
• IgA form the first line of defense for mucosal surfaces, breast milk, saliva, and tears. They gather the bound antigens and expel them with your mucus or other body fluids.
• IgE trigger allergies and protect against parasites. Small amounts are found in your skin, lungs, and mucosal membranes.
• IgD kick-start your immune response during early infection. Instead of circulating, IgD bind to B cells to signal the release of IgM.
Each antibody guards against its target antigen and a sundry of them can be found throughout your body. As such, they play a vital role in your body’s ability to defend against many types of illness and disease.
How are antibodies and antigens different?
Your immune system springs into action when an infectious agent such as SARS-CoV-2, the virus that causes COVID-19, invades your body.
Antigens take part in the breach and set off an immune response. Antibodies form and launch a counterattack by binding and disarming the antigen.
The site on the antibody that binds to the antigen is called the paratope, and is found at the tips of the Y. These latch onto and interact with a site on the antigen known as the epitope. Your immune system is equipped to spot a wide array of antigens due to the equally vast flux of paratopes.
Each B cell can only make antibodies against one specific epitope. But after being activated, your B cells will replicate to make clones that secrete that antibody.
How are antigens and antibodies used in vaccinations?
Traditional Vaccines
Vaccines include weakened or inactive parts of antigens to compel your B cells to make targeted antibodies that fight the disease. Newer vaccines house the blueprint for making antigens instead of using antigen components but work much in the same way.
Vaccines boost circulating antibody levels against a certain antigen. Your B cells respond as if a naturally occurring antigen has attacked your body. They replicate to form a legion of cells that react to the antigens in the vaccine.
The cloned cells transform into plasma cells that produce antibodies, as well as memory B cells which lie dormant for periods of time until they are called to action. This is your immune system’s primary response to the antigens in the vaccine.
Memory B cells recall which antigens you were vaccinated against. If you are exposed to the antigen again, they wake up and quickly make more plasma cells trained to make the antibodies needed to destroy the antigen.
This secondary immune response or antigen assault is much faster and fierce.
COVID-19 Vaccines
Messenger ribonucleic acid (mRNA) COVID-19 vaccines carry the blueprint for making a SARS-CoV-2 antigen in the form of RNA, a molecule that stores the genetic code that all living cells use as a guide for making their protein parts.
Once the mRNA code is transcribed, it yields the lab-created antigen that sets off an immune response.
mRNA COVID-19 vaccines prompt antibodies against the spike proteins that sit on the SARS-CoV-2 outer surface. These proteins are vital to the virus as they are used to enter your cells. Therefore, the virus cannot alter them as a means of eluding immune detection.
Your antibodies latch onto these spike proteins, keeping the virus from binding to your healthy cells and causing infection.
What is antigen testing?
Antigen tests are used to quickly determine if an active infection is present by looking for certain proteins on the surface of a virus such as SARS-CoV-2.
SARS-CoV-2 antigen tests are often performed at the point of care, such as your doctor’s office, instead of being sent to a lab for analysis. Once your healthcare provider swabs your nose or throat, it takes about 15 minutes to get test results back.
SARS-CoV-2 antigen testing is less sensitive than nucleic acid amplification testing (NAAT), which most often uses a reverse-transcription polymerase chain reaction (RT-PCR) assay to support a COVID-19 diagnosis. Thus, false-negative results are more likely to occur with antigen testing.
Antigen test results are most accurate when your chance of contracting SARS-CoV-2 is high. A positive result is more likely to be correct if it is performed about a week after you likely contracted the virus.
During this timeframe, the coronavirus replicates quickly within your body and your viral load tends to peak.
When NAAT cannot be used or test turnaround times take too long to be useful in clinical care settings, the World Health Organization (WHO) recommends using SARS-CoV-2 antigen tests with a minimum:
- • 80 percent or higher sensitivity (true positive rate), which is the test’s ability to correctly produce a positive result
• 97 percent or higher specificity (true negative rate), which is the test’s ability to correctly produce a negative result
In these cases, WHO advises that antigen testing be performed within the first 5 to 7 days of symptoms. Most antigen tests can detect SARS-CoV-2 infection within this window while a few can detect it up to 12 days after symptoms start.
Although negative antigen test results should be confirmed with NAAT in most cases, this may not be needed if you meet all of the following:
- • Do not have COVID-19 signs and symptoms
• Have not had close contact to a person who tested positive for COVID-19
• Live and work in a community where SARS-CoV-2 infection rates are low
Be sure to confirm with your doctor or healthcare provider whether NAAT is needed to confirm your SARS-CoV-2 antigen test results.
Along with providing quick access to COVID-19 testing, antigen testing may be useful in:
- • Community settings where outbreaks have occurred
• High-risk group settings where repeat antigen testing helps quickly identify people living with SARS-CoV-2, which allows for prompt isolation and care measures to be taken
Negative antigen tests do not need to be confirmed with NAAT in settings where serial antigen testing is performed.
What is antibody testing?
Antibody tests cannot be used to diagnose an illness, but they can show whether you have had a previous infection by detecting antibodies in your blood. These can be performed at the point of care or at specialized labs and include tests such as:
- • Enzyme-linked immunofluorescent assay
• Western blot
• Immunoelectrophoresis
IgM usually appears on your antibody test about 1 to 2 weeks after you contract a virus or other agent that causes illness and disappears from your blood after a few months.
Your immune system forms IgG about two weeks after the agent is acquired and antibody tests can often detect it in your blood long after you have recovered from your illness.
COVID-19 Antibody Testing
Blood tests can detect antibodies to SARS-CoV-2, and may help identify if you acquired SARS-CoV-2 in the past. They can also detect this status if you have had COVID-19 symptoms for 3 to 4 weeks.
The Infectious Diseases Society of America (IDSA) recommends using IgG or total antibody tests to detect SARS-CoV-2 due to their higher specificity.
If you have had a spike protein COVID-19 vaccine, IDSA also suggests using a test that detects antibodies to antigens other than the SARS-CoV-2 spike protein.
The amount of time it takes for SARS-CoV-2 antibodies to be detected varies with each test. It can also take a few days to weeks for your body to form antibodies.
A 2021 study published in the Journal of Infectious Diseases found that most participants still had detectable SARS-CoV-2-specific antibodies close to a year after COVID-19 infection close to a year after COVID-19 infection.
This supports previous findings also published in 2021. The authors noted that this immune response could be detected for up to eight months after infection with the coronavirus.
What can you take away
Antigens compel your immune system to launch an antibody response.
Specific antibodies detect specific antigens. This mean each antibody wages war against one target antigen. Once they detect the antigen, they bind and neutralize it.
This information is stored in your immune system’s long-term memory. If it finds the same antigen trying to attack your body again, it launches a rapid-fire assault against the antigen again.
The distinct properties and functions of antigens and antibodies are channeled to create tests and vaccines that help detect and combat illness and disease.
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A version of this article has been published on Healthline.A
