Antigen: What It Is, Function, Types, & Testing - Cleveland Clinic

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.

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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.

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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!

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