Antigen

What is an antigen?

An antigen is any kind of marker — like a protein or string of amino acids — that your immune system can recognize. If this definition seems a little vague to you, you’re not alone. Let’s dig in further.

Antigens are usually proteins or sugars (polysaccharides) found on the outside of things like cells or viruses. Each has a unique shape that your immune system reads like a nametag to know whether it belongs in your body.

Antigens exist on viruses, bacteria, allergens, parasites, proteins, tumor cells and normal cells in your own body. You might hear your own body’s antigens referred to as “self” and viruses, bacteria and other harmful antigens referred to as “non-self.” This means that your body recognizes your own cells as you, but other antigens as intruders.

What is the difference between an antigen and an antibody?

Antigens are markers that tell your body that something is foreign. Your immune cells make antibodies to recognize and destroy harmful antigens. In fact, you can think of antigens as antibody generators.

Antibodies are very specific to the antigens they recognize and destroy. They fit onto the antigen like a key to a lock.

What are the types of antigens?

There are several types of antigens, categorized by where they come from. These include exogenous antigens, endogenous antigens, autoantigens and tumor antigens.

Exogenous antigens

Exogenous antigens come from foreign substances that can enter your body through your nose, your mouth or cuts in your skin. These include viruses, bacteria, pollen, parasites and fungi.

Endogenous antigens

Endogenous antigens exist on cells inside your body. They tell your immune system that they are either friendly (“self”) or harmful. These include cells that are infected with bacteria or a virus that mark themselves to be destroyed by the immune system. Red blood cell antigens and special markers that your body recognizes as “self” (HLAs) are also endogenous antigens.

Autoantigens

Autoantigens are markers on cells inside your body that your immune system attacks even though they shouldn’t. Autoantigens cause autoimmune diseases.

Tumor antigens

Tumor antigens are markers on the surface of tumors. You might hear these called tumor-associated antigens (TAA), tumor-specific antigens (TSA), neoantigens or oncogenic antigens.

These antigens are sometimes normal parts of a cell that act differently in tumor cells (for example, a tumor cell might make much more of a particular protein than a normal cell). Other times, they come from mutations in the tumor’s genes or from a virus inside the tumor cells.

What are antigen-presenting cells?

Antigen-presenting cells help the immune system mount its attack. There are three types of antigen-presenting cells in your body: macrophages, dendritic cells and B cells.

One of their jobs is to act like a detective, showing the other cells of the immune system suspects they think are invading your body. (In fact, you’ll sometimes hear them called “professional” antigen-presenting cells.)

When one of these specialized cells comes across an antigen, it devours the antigen, breaks it apart and displays parts of the antigen on its cell surface. This serves as a kind of “wanted poster” for T cells. The T cells inspect the bits of antigen to decide if they recognize the invader. If that particular T cell’s unique receptor is a match for the antigen, it alerts the rest of the immune system to attack.

What happens when an antigen enters your body?

Your body defends itself against harmful antigens with chemicals called antibodies. When an antigen enters your body, the B-cells of your immune system inspect it.

B-cells have special parts (receptors) that test the antigen to see if they fit together, like a lock and key. If it’s a fit, the B-cell makes chemicals called antibodies that all have the same shape to recognize the antigen.

An antibody’s job is to find antigens that fit the specific shape on their surface. Antibodies lock on (bind) to antigens so that your immune system can destroy them.

Once your immune system has seen an antigen, it has special cells that remember it, allowing your body to create a faster and more effective response if it invades again — this is called immunological memory. Unfortunately, antigens change shape when a harmful substance mutates, and your immune system may not be able to lock onto it to defend itself effectively anymore. Imagine what would happen if the locks on your house changed every time you left home!

What is an antigen test?

Antigen tests examine your blood, spit, pee, poop or other bodily fluids for specific markers of a disease. They’re usually used to find out if you have a viral illness such as the flu or COVID-19 or to monitor cancer treatment. They can also help screen organ donors and recipients.

How does an antigen test work?

How an antigen test works depends on what kind of antigen you’re looking for.

For a virus, usually your healthcare provider will swab your nose or your throat with a long stick with a soft, spongy tip. Then they’ll test the tip of the stick to see if it has antigens for the virus they’re looking for. You can also do some viral antigen tests at home. You can often have the results of these tests in as little as 15 minutes.

For other types of antigen tests, like those that look for cancer markers or tissue compatibility, your healthcare provider takes a sample of your blood, pee or poop. Then they’ll send the sample to a lab to be tested for the antigen they’re looking for.

Are antigen tests accurate?

The accuracy of antigen tests depends on what kind of antigen you’re looking for and how you test for it. Rapid antigen tests for viral infections, for instance, are most accurate when used within a few days of the start of your symptoms. This is when the largest amount of virus is present in your body and gives the test the best chance of detecting it.

What are some common types of antigen tests?

Some common antigen tests include:

• Carcinoembryonic antigen (CEA) test: CEA is typically found in low levels in adults. Certain conditions, including cancer, can cause high levels of CEA in your blood. Your healthcare provider might monitor your levels of CEA in many cancers to understand if treatment has been effective.
• Hepatitis B surface antigen test (HBsAg): An HBsAg is a blood test that providers use in combination with other tests to diagnose hepatitis B.
• Human leukocyte antigen (HLA) test: All of our cells except red blood cells have a combination of HLAs that’s unique to us. Providers test recipients of organs or stem cells and their donors to see if they have similar HLAs. Recipients also undergo tests to make sure they don’t have antibodies to a donor’s HLAs, which would cause their body to attack the organ or cells from the donor (graft versus host disease). Many transplants are much more likely to be successful if the recipient and the donor have similar HLAs. They don’t have to be a perfect match.
• Prostate-specific antigen test (PSA): PSA levels can be higher in people with prostate cancer or less serious prostate conditions. A PSA test may can happen as a part of a routine screening with your healthcare provider.
• SARS-Cov-2 or COVID-19 antigen test: Sometimes called a rapid test, this looks for antigens to SARS-CoV-2, the virus that causes COVID-19, in a sample taken from your nose or throat. You can do this test at home with a test kit or with a healthcare provider.

What are blood group antigens?

You’re probably familiar with blood types — A, B, AB and O. These letters actually represent antigens on the surface of red blood cells. You can be born with A or B antigens, both (AB blood) or neither (O blood). This is called the ABO blood group.

The “+” and “-” signs in your blood type indicate the presence or absence of other antigens called the Rhesus factor. You might also hear the terms “Rhesus positive” or “Rhesus negative.”

If you have a particular antigen on your blood cells, your body knows it’s friendly and won’t make antibodies against it. Otherwise, your immune cells will attack blood with that antigen.

In other words, if you have type A blood, your antibodies will attack blood with B antigens and vice-versa. If you have AB blood, your body sees both antigens as friendly and won’t attack either. If you have O blood, you have antibodies to both A and B antigens.

This is why blood type is important. If you receive blood that is incompatible with your type, it can cause a very serious immune reaction.

What are human leukocyte antigens (HLA)?

Human leukocyte antigens (HLAs) are special markers that identify your cells as “self,” or belonging to your body. All of your cells have HLAs except your red blood cells.

HLAs can be thought of as similar to your blood type, but they’re a lot more complex. For instance, we all have many HLAs, so it’s very unlikely to find someone else with the exact same combination.

You can have antibodies to HLAs, causing an autoimmune disease if they happen to be your own. If you need a stem cell transplant, you'll need to undergo HLA tests to find someone who is a close match. This is usually a close relative, like a parent, sibling or child.

A note from Cleveland Clinic

Wouldn’t it be nice if everything we encountered in our jobs came with a label to tell us if it’s friendly or harmful? Antigens are handy tools that allow our immune system to do just that.

Your healthcare provider may order antigen tests for routine screenings, to monitor an ongoing condition or to determine whether you have a virus. Be sure to ask your provider any questions you have about how they'll perform a test or why they're performing it.