Review
What is an antigen?
An antigen is any kind of marker - such as a protein or an amino acid sequence - thatimmune systemcan recognize. If this definition seems a little confusing to you, you're not alone. Let's keep digging.
Antigens are usually proteins or sugars (polysaccharides) found on the outside of cells or viruses. Each has a unique shape that your immune system reads like a badge to know if it belongs to your body.
Antigens are found in viruses, bacteria, allergens, parasites, proteins, cancer cells, and normal body cells. You may hear your body's antigens called "self" and viruses, bacteria and other harmful antigens called "foreign". This means that your body recognizes your own cells as you, but other antigens as invaders.
What is the difference between an antigen and an antibody?
Antigens are markers that tell your body that something is foreign. Your immune cells createantibodiesrecognize and destroy harmful antigens. In fact, you can think of antigens asInsteadbodygencontractor
Antibodies are very specific to the antigens they recognize and destroy. They fit the antigen like a key to a lock.
What are the types of antigens?
There are several types of antigens, classified according to their origin. These include exogenous antigens, endogenous antigens, self-antigens and tumor antigens.
Exogenous antigens
Exogenous antigens come from foreign substances that can enter the body through the nose, mouth, or skin cuts. These include viruses, bacteria, pollen, parasites and fungi.
Endogenous antigens
Endogenous antigens are present in your body's cells. They tell your immune system that they are either friendly ("self") or harmful. This includes cells that have been infected with bacteria or viruses, marking them for destruction by the immune system. Red blood cell antigens and specific markers that the body recognizes as "self" (HLA) are also endogenous antigens.
Autoantygeny
Self-antigens are markers on cells in your body that your immune system attacks when it shouldn't. Autoantigenic causeautoimmune diseases.
Cancer antigens
Tumor antigens are markers on the surface of a tumor. You may hear them as tumor-associated antigens (TAAs), tumor-specific antigens (TSAs), neoantigens, or oncogenic antigens.
These antigens are sometimes normal parts of the cell that act differently on cancer cells (for example, a cancer cell may produce much more of a certain protein than a normal cell). In other cases, they come from mutations in cancer genes or from viruses inside cancer cells.
What are antigen presenting cells?
Antigen-presenting cells help the immune system launch its attack. There are three types of antigen-presenting cells in the body: macrophages, dendritic cells, and B cells.
One of their jobs is to act as detectives, pinpointing other immune system cells that they suspect are attacking your body. (In fact, they are sometimes heard as "professional" antigen-presenting cells.)
When one of these specialized cells encounters an antigen, it engulfs the antigen, breaks it down, and displays parts of the antigen on the surface of the cell. This serves as a kind of "wanted notice" for the T cells. The T cells examine the antigen fragments to decide if they recognize the invader. If the unique receptor of that particular T cell matches the antigen, it alerts the rest of the immune system to an attack.
What happens when an antigen enters the 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 examine it.
B cells have special parts (receptors) that test the antigen to see if they match, like a lock and a key. When attacked, the B cell produces chemicals called antibodies that have the same shape to recognize the antigen.
The job of antibodies is to find antigens that match a certain shape on their surface. Antibodies block (bind to) antigens so that the immune system destroys them.
Once your immune system detects an antigen, it has special cells to remember it, allowing the body to react more quickly and efficiently if it attacks again - this is called immune memory. Unfortunately, antigens change form when the harmful substance mutates, and your immune system may no longer be able to attach itself to it to defend itself effectively. Imagine what would happen if the locks on your house were changed every time you left the house!
Diagnosis and tests
What is an antigen test?
Antigen tests test blood, sputum, urine, feces or other body fluids for specific disease markers. They are usually used to check if you have a viral illness such as the flu or COVID-19, or to monitor cancer treatment. They can also help with revisionorgan donors and recipients.
How does the antigen test work?
The way an antigen test works depends on the type of antigen you are looking for.
In the case of a virus, the doctor usually cleans the nose or throat with a long stick with a soft sponge tip. They will then test the tip of the swab to see if it contains the antigens of the virus they are looking for. Some viral antigen tests can be done at home. You can often get the results of these tests in as little as 15 minutes.
For other types of antigen tests, such as those that look for tumor markers or tissue compatibility, your doctor takes a blood, urine, or stool sample. They will then send the sample to a lab to test for the antigen they are looking for.
Are antigen tests accurate?
The accuracy of antigen testing depends on what type of antigen you are looking for and how you test it. For example, rapid antigen tests for detecting viral infections are most accurate when used within a few days of the onset of symptoms. This is when there are the most viruses in the body and gives the test the best chance of detecting it.
What are the common types of antigen tests?
Some common antigen tests include:
- Carcinoembryonic antigen (CEA) test.: CEA is usually at low levels in adults. Some conditions, including cancer, can cause high levels of CEA in the blood. Your healthcare professionalcan monitor CEA levelsin many types of cancer to understand if the treatment was effective.
- Hepatitis B surface antigen (HBsAg) test.: HBsAg is a blood test that doctors use in conjunction with other diagnostic testsHepatitis B.
- Human leukocyte antigen (HLA) test.: All of our cells, except red blood cells, have an HLA combination unique to us. Suppliers screen recipients of organs or stem cells and their donors to see if they have similar HLA. Recipients are also tested to make sure they don't have antibodies against the donor's HLA, which could cause their body to attack the donor's organ or cells.graft versus host disease). Many transplants have a much better chance of success if the recipient and donor have similar HLA. They don't have to fit perfectly.
- Prostate-specific antigen (PSA) test.: PSA levels may be higher in people with prostate cancer or less serious prostate conditions. A PSA test can be done as part of a routine check-up with your doctor.
- SARS-Cov-2 or COVID-19 test and antigen: Sometimes called a rapid test, it looks for the antigens of SARS-CoV-2, the virus that causes itCOVID 19, in a sample taken from the nose or throat. You can perform this test at home with a test kit or at your doctor's office.
Frequently asked questions
What are blood group antigens?
You probably knowblood group- A, B, AB and O. These letters actually represent antigens on the surfaceRed bloodcells. 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 group indicate the presence or absence of other antigens called Rhesus factor. You may also hear the terms "Rhesus positive" or "Rhesus negative".
If you have a specific antigen on your blood cells, your body knows it's friendly and doesn't make antibodies against it. Otherwise, your immune cells will attack the blood with this antigen.
In other words, if you have blood type A, your antibodies will attack blood with B antigens, and vice versa. If you have AB blood, your body perceives both antigens as friendly and will not attack either of them. If you have O blood, you have antibodies to both A and B antigens.
That's why blood type is important. If you receive blood that does not match your type, it can cause a very serious immune reaction.
What are human leukocyte antigens (HLA)?
Human leukocyte antigens (HLA) are special markers that identify your cells as "I" or as belonging to your body. All your cells have HLA except red blood cells.
HLA can be thought of as similar to blood type, but they are much more complex. For example, we all have multiple HLAs, so it's very unlikely we'll find someone else with the exact same combination.
You may have HLA antibodies causing an autoimmune disease if they are yours. If you need a stem cell transplant, you will need to undergo HLA testing to find a match. This is usually a close relative, such as a parent, sibling or child.
Message from the clinic in Cleveland
Wouldn't it be nice if everything we come across at work had a label telling us if it's friendly or harmful? Antigens are useful tools that allow our immune system to do just that.
Your doctor may order antigen tests for routine testing, monitoring your current condition, or determining if you have the virus. Be sure to ask your provider any questions about how they will conduct the test or why they will perform it.
FAQs
What is an antigen and its function? ›
An antigen is any substance that causes your immune system to produce antibodies against it. This means your immune system does not recognize the substance, and is trying to fight it off. An antigen may be a substance from the environment, such as chemicals, bacteria, viruses, or pollen.
What is antigen and types of antigen? ›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.
What type of test is antigen? ›What is an antigen test? An antigen test is used to identify people with active infections. This test is a diagnostic tool. A positive result generally means a person currently has COVID-19 and can spread the virus to others.
What is the function of an antigen quizlet? ›Define antigen. Antigens are foreign molecules that are recognized by the immune system. They can bind to antigen-specific receptors (antibodies and T cell receptors). Antigens that do not bind, do not induce an immune response.
What is in an antigen? ›In general, antigens are composed of proteins, peptides, and polysaccharides. Any portion of bacteria or viruses, such as surface protein, coat, capsule, toxins, and cell wall, can serve as antigens.
What is an example of a antigen? ›Examples include parts of or substances produced by viruses or microorganisms (such as bacteria and protozoa), as well as substances in snake venom, certain proteins in foods, and components of serum and red blood cells from other individuals.
What are the main types of antigen? ›There are three main types of antigen
The three broad ways to define antigen include exogenous (foreign to the host immune system), endogenous (produced by intracellular bacteria and virus replicating inside a host cell), and autoantigens (produced by the host).
There are three types of antigens, classified based on where they are produced. Self-antigens, or autoantigens, are produced in the body's own cells; endogenous antigens are produced in intracellular bacteria or viruses; exogenous antigens are produced outside the body and are foreign to the immune system.
How do antigen tests work? ›Antigen test.
This COVID-19 test detects certain proteins in the virus. Using a nasal swab to get a fluid sample, antigen tests can produce results in minutes. Others may be sent to a lab for analysis. A positive antigen test result is considered accurate when instructions are carefully followed.
Antigen tests produce results quickly (within minutes), and most can be used at the POC or at home. Most self-tests, or at-home tests, are antigen tests. Antigen tests are better at detecting a SARS-CoV-2 infection when someone has COVID-19 symptoms compared to if they do not.
Is antigen a viral test? ›
There are two main types of viral tests: Polymerase Chain Reaction (PCR) tests. Antigen tests.
What is the function of antigen receptors? ›The B cell antigen receptor (BCR) is a multimeric complex of cell surface–expressed proteins that fulfills two critical roles upon detection of extracellular antigens: first, it induces signal transduction cascades that initiate cellular activation; second, it internalizes bound antigen and targets it to intracellular ...
What is the function of antigen Wikipedia? ›In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response.
What is an antigen simple answer? ›(AN-tih-jen) Any substance that causes the body to make an immune response against that substance. Antigens include toxins, chemicals, bacteria, viruses, or other substances that come from outside the body. Body tissues and cells, including cancer cells, also have antigens on them that can cause an immune response.
What is an antigen test quizlet? ›-Type of testing used most in diagnosis of viral diseases. -Most often use blood serum & look for antibody against a specific antigen or the virus itself (antigen)
What is an antigen result? ›There are 3 types of results for a COVID-19 (coronavirus) PCR or antigen test: positive (COVID-19 found) negative (COVID-19 not detected) indeterminate, invalid or inhibitory.
What are three antigens examples? ›Antigens are known to trigger the immune system. They can be endogenous, exogenous, or autoantigens. Examples of antigens are pollen, bacteria, parasitic worms, and viruses.
Which is the most important antigen? ›The ABO blood group antigens remain of prime importance in transfusion medicine—they are the most immunogenic of all the blood group antigens.
How many antigens are there? ›Today, more than 700 blood group antigens are known to exist. An antigen (immunogen) is any substance that can induce a specific immunological response, i.e., cause the production of antibody.
How many different antigens are there? ›There are currently 44 recognised blood group systems containing 354 red cell antigens (December 2022). The 44 systems are genetically determined by 49 genes.
What are the 2 main antigens? ›
The antigens expressed on the red blood cell determine an individual's blood group. The main two blood groups are called ABO (with blood types A, B, AB, and O) and Rh (with Rh D-positive or Rh D-negative blood types).
What blood types are antigens? ›| Name of Blood Group | Antigens present on the red cell surface | ABO antibodies present in the plasma |
|---|---|---|
| Type O | nil | anti-A and anti-B |
| Type A | A antigen | anti-B |
| Type B | B antigen | anti-A |
| Type AB | A and B antigens | nil |
Acquired immunity is immunity that develops with exposure to various antigens. Your immune system builds a defense against that specific antigen. Passive immunity is due to antibodies that are produced in a body other than your own.
What are the 7 functions of antibodies? ›- Activation of complement. ...
- Binding Fc receptors. ...
- 3.1 Opsonization promotes phagocytosis. ...
- 3.2 Mediated allergic reactions. ...
- 3.3 Antibody-dependent cellular cytotoxicity, ADCC effect. ...
- Through the placenta. ...
- Immune regulation.
Antibody serology tests check for the presence or level of specific antibodies in the blood. Antibodies are proteins that your immune system makes to fight foreign substances. These substances are often pathogens, (disease-causing germs) such as viruses and bacteria.
What is the difference between antigen test and PCR test? ›Molecular tests, such as polymerase chain reaction (PCR) and other nucleic acid amplification tests (NAATs) tests, which detect genetic material called RNA from the virus. Antigen tests, often referred to as rapid tests or, for some, at-home or self tests, which detect proteins called antigens from the virus.
What is the difference between an antigen and an antibody? ›Antigens allow your body to create a defense against future invaders. Antibodies circulate in your body once created to identify, attack, and destroy the same type of antigens if they enter the body again.
How accurate are antigen tests? ›Be aware that at-home COVID-19 antigen tests are less accurate than molecular tests. COVID-19 antigen tests may not detect the SARS-CoV-2 virus early in an infection, meaning testing soon after you were exposed to someone with COVID-19 could lead to a false-negative result, especially if you don't have symptoms.
Does Covid show up on antigen tests? ›Antigen tests are quicker but less reliable than a PCR test. In some cases, they may not detect the virus. You can buy antigen tests in pharmacies and supermarkets.
How do you self test an antigen? ›gently into 1 of their nostrils until you feel some resistance. inside of their nostril in a circular motion 10 times. If you did not swab the tonsils, swab both nostrils, making 10 complete circles in each nostril. For best performance, test the swab as soon as possible after wiping your nostril.
Is the COVID virus an antigen? ›
COVID-19 antigen tests are designed for the rapid diagnoses of active infection primarily by detecting the nucleocapsid protein antigen of the SARS-CoV-2 virus (the virus that causes COVID-19) from nasal swabs or similar clinical specimens.
How much is an antigen test? ›Gordon said that starting this Thursday, PRC's molecular laboratories at the PLMC Building in Mandaluyong City and the PRC Port Area Offices will be offering COVID-19 antigen tests for Php400. 00.
How accurate is the rapid Covid test? ›In people with confirmed COVID-19, antigen tests correctly identified COVID-19 infection in an average of 73% of people with symptoms, compared to 55% of people without symptoms. Tests were most accurate when used in the first week after symptoms began (an average of 82% of confirmed cases had positive antigen tests).
What do Covid antigens mean? ›COVID-19 antigen tests are designed for the rapid diagnoses of active infection primarily by detecting the nucleocapsid protein antigen of the SARS-CoV-2 virus (the virus that causes COVID-19) from nasal swabs or similar clinical specimens.
What is difference between antibody and antigen? ›Antigen vs antibody
An antigen is a foreign substance that enters your body. This can include bacteria, viruses, fungi, allergens, venom and other various toxins. An antibody is a protein produced by your immune system to attack and fight off these antigens.
(Learn more about what a positive COVID-19 test means here.) Antigens get a bad reputation, but they play an important role. They help your immune system detect threats, which helps keep you safe.
What do antigen tests detect? ›At-home over-the-counter (OTC) COVID-19 antigen tests, often referred to as rapid tests, detect proteins called antigens from SARS-CoV-2, the virus that causes COVID-19.
How an antigen test works? ›How do most COVID-19 antigen tests work? Antigen tests are designed based on lateral flow assays; an inexpensive technology that uses paper-based platforms to detect chemical analytes within short durations of time.
What is the difference between Covid antibody and antigen? ›A positive antigen test means that the person being tested has an active COVID-19 infection. A positive antibody test means that the person being tested was infected with COVID-19 in the past and that their immune system developed antibodies to try to fight it off. appears on tests first, usually within 1-2 weeks.
Which is the effect of antigen in an ill person? ›The presence of antigens rouses your body's illness-fighting white blood cells, called lymphocytes. This presence of antigens causes white blood cells to make cells called antibodies to fight against the antigens.
What are the 5 types of antibodies? ›
There are 5 types of heavy chain constant regions in antibodies (immunoglobulin) and according to these types, they are classified into IgG, IgM, IgA, IgD, and IgE. They are distributed and function differently in the body.
Can antigens affect your immune system? ›If an antigen gets past these barriers, it is attacked and destroyed by other parts of the immune system. Acquired immunity is immunity that develops with exposure to various antigens. Your immune system builds a defense against that specific antigen.
Can I test negative on antigen? ›Positive results are very accurate and reliable. However, in general, antigen tests are less likely to detect the virus than PCR tests, especially when symptoms are not present. Therefore, a single negative antigen test cannot rule out infection.
What is the difference between a swab and an antigen test? ›Antigen tests produce results more quickly than other tests. They are given with a nasal swab. A positive antigen test result means a person currently has COVID-19. Antigen tests are not as sensitive as molecular tests, no matter where they are performed (such as home, clinic, pharmacy).