Have you ever thought that our body has its own defense system to fight off diseases from outside? Isn't it amazing? So, for this defense system to work, it needs to have a way to recognize "who is friend and who is enemy." That's where these "antigens" come in. Simply put, these are the signals that tell our body's defenses, "Here's someone to catch!"
What exactly is an antigen?
Think of your body as a big country. It's like everyone who enters this country has an ID card that identifies them. Antigens are actually proteins or polysaccharides. They are on the outside of things like cells, viruses, and bacteria. It's like a label with a name on it. Our immune system reads this label to decide whether this is something that belongs to the body or an invader that has come from outside.
Now look, these antigens are not only found in things like viruses, bacteria, pollen, parasites, and cancer cells, but also in our own healthy cells. We call the antigens in our own bodies "self." Antigens in harmful things like viruses and bacteria are called "non-self." This means that our body recognizes its own cells and acts against those that come from outside. Very clever, isn't it?
Do you know the difference between antigen and antibody?
These two may seem a bit confusing, but it's very simple.
Antigens are those markers , as I mentioned earlier. These markers are what tell our body, "Here comes someone from outside!"
Antibodies are special proteins made by our immune cells. They are made to recognize and destroy harmful antigens. In fact, the word antigen comes from the idea of "antibody generators," meaning "things that help make antibodies."
Antibodies are very special. Each antibody can only recognize one specific antigen. It's like a key and a lock. Only the right key can open the lock. That's how these two work.
What types of antigens are there?
Antigens can be divided into several types depending on how they come from. Let's see what they are.
1. Exogenous Antigens
These are things that enter our bodies from the outside. Just think, these can enter your body through your nose, mouth, or even through a skin wound.
- Viruses
- Bacteria
- Pollen
- Parasites
- Fungi
All of these belong to this type.
2. Endogenous Antigens
These antigens are found on cells inside our bodies. They tell our immune system that "I am your friend" (self) or "I am now a bad person, destroy me" (harmful). For example, our own cells that have been infected with a bacteria or virus put on such marks, telling the immune system to destroy them. The antigens on our red blood cells and the special markers that recognize our body as "mine" called ``(HLAs)`` (Human Leukocyte Antigens) also belong to this type.
3. Autoantigens
This is a bit of a confusing situation. These are the signs of our own healthy cells. However, sometimes our immune system makes a mistake and attacks these healthy cells, thinking they are "enemies." That's when autoimmune diseases develop.
4. Antigens found in cancer cells (Tumor Antigens)
These are special markers on the surface of cancer cells. You may also hear them called `(Tumor-associated antigens - TAA)`, `(Tumor-specific antigens - TSA)`, `(Neoantigens)` or `(Oncogenic antigens)`.
Sometimes these are the same parts as a normal cell, but in a cancer cell they work differently (for example, a cancer cell may make much more of a certain protein than a normal cell). Other times, these antigens are produced by mutations in the cancer cell's genes or by a virus inside the cancer cell.
What are Antigen-Presenting Cells (APCs)?
This is a very important group. These cells are what help the immune system say, "Here comes the fight!" We have three types of these cells in our bodies: macrophages, dendritic cells, and B cells.
Simply put, they work like clever detectives. They point out what they think are invaders to other immune cells. That's why they're sometimes called "professional antigen-presenting cells."
When this special cell encounters an antigen, it devours it, breaks it into small pieces, and then displays some of the pieces on its cell surface. This is like a ``wanted poster'' for T cells . T cells come and examine these pieces of antigen to see if they recognize this invader. If the antigen matches a specific receptor on a T cell, that T cell tells the rest of the immune system to ``attack!''
What happens when an antigen enters the body?
Your body protects itself from harmful antigens from outside by using chemicals called antibodies . When an antigen enters your body, the B-cells in your immune system examine it carefully.
B-cells have special receptors that check to see if the antigen is a good match – like the key and lock mentioned earlier. If there is a match, the B-cell starts making antibodies. Each antibody it makes has a unique shape that can recognize that antigen.
The job of an antibody is to find an antigen that matches the special shape on its surface. Antibodies go and bind to these antigens. Then your immune system can destroy those harmful things.
Once your immune system has encountered an antigen, it has special cells that remember it. This is called immunological memory . This is why if the same invader comes back, the body can respond much more quickly and efficiently.
But there's a little problem. Sometimes these harmful things mutate, meaning their antigens change shape. Then our immune system can't recognize them, just like if all the bugs in your house had changed when you left home!
What is an antigen test?
Antigen tests look for specific markers (antigens) in your body fluids, such as blood, saliva, urine, and stool, that are associated with a particular disease. These are commonly used to find out if you have the flu or a viral illness like COVID-19, or to see how cancer treatment is going. They can also help screen people who are going to donate organs or receive transplants.
How is an antigen test performed?
This depends on what kind of antigen you are looking for.
- For a virus: A doctor will usually swab your nose or throat with a small, soft-tipped swab. They will then test the swab to see if it contains antigens from the virus they are looking for. Some viral antigen tests can be done at home (e.g., COVID-19 Rapid Antigen Test). These tests can provide results in as little as 15 minutes.
- For other antigen tests (such as cancer markers, tissue matching): The doctor will take a sample of your blood, urine, or stool. They will then send it to a lab to check for the antigen they are looking for.
How accurate are antigen tests?
This also depends on what antigen you are looking for and how you are testing. For example, rapid antigen tests for viral infections are most accurate if they are done within a few days of your symptoms starting. This is because that is when the virus is most abundant in the body, making it easier for the test to pick up.
Several types of commonly used antigen tests
Here are some tests you may have heard of, or had to do:
- Carcinoembryonic antigen (CEA) test: CEA is normally present in low levels in adults. However, in some conditions, such as cancer, the level of CEA in the blood can be elevated. Doctors monitor this level of CEA to see if cancer treatment is working.
- Hepatitis B surface antigen (HBsAg) test: The HBsAg test is a blood test that is used in conjunction with other tests to diagnose hepatitis B.
- Human leukocyte antigen (HLA) test: Every cell in our body, except our red blood cells, has its own set of HLAs. When transplanting organs or stem cells, the donor and recipient are checked to see if they have the same HLAs. The recipient is also checked to see if they have antibodies against the donor's HLAs. If they do, the body can attack the new organ (this is called graft versus host disease). For most transplants to be successful, the donor and recipient need to have a good match in HLAs. But they don't have to be exactly the same.
- Prostate-specific antigen (PSA) test: PSA levels can be elevated in men with prostate cancer or in those with less serious prostate problems. This PSA test can be done as part of a routine medical exam.
- SARS-CoV-2 or COVID-19 Antigen Test: This is sometimes called a "rapid test." It looks for the presence of antigens from the virus that causes COVID-19, SARS-CoV-2, in a sample taken from your nose or throat. This can be done at home with a test kit or by a doctor.
What are blood group antigens?
You may have heard of blood types A, B, AB, and O. These letters actually refer to the types of antigens on the surface of your red blood cells. You can be born with either A or B antigens, or both (blood type AB), or neither (blood type O). This is called the ABO blood group system.
The "+" or "-" sign in your blood type indicates whether you have another antigen called the Rhesus factor or Rh factor . You may have also heard of being "Rhesus positive" or "Rhesus negative."
If your blood cells have an antigen, your body knows it's "friendly" and won't make antibodies against it. Otherwise, your immune cells will attack the blood that has that antigen.
In other words, if your blood type is A, your antibodies will attack blood with the B antigen. And the same goes for the other way around. If your blood type is AB, your body sees both antigens A and B as "friendly" and will not attack them. If your blood type is O, you have antibodies against both antigens A and B.
That's why it's so important to match your blood type when donating blood. If you're given blood that doesn't match your blood type, it can cause a severe immune reaction.
A little more about Human Leukocyte Antigen (HLA)
HLAs are special markers that identify your cells as "self," meaning they belong to your body. All cells except your red blood cells have HLAs.
You might think of `(HLAs)` as blood types, but they are much more complex than that. For example, we all have many `(HLAs)`, so it is very rare to find another person with the exact same `(HLA)` combination.
You can develop antibodies against your own HLAs, which can lead to an autoimmune disease. If you need a stem cell transplant, you will need to have HLA testing to find a match. In this case, the best match is usually a close relative, such as a mother, father, brother, or sister.
Finally, things to remember
Wouldn't it be great if everything we encounter on our way to work had a label saying "this is good, this is bad"? Well, antigens are a very useful tool that helps our immune system do exactly that.
Your doctor may order antigen tests for routine checkups, to monitor an ongoing illness, or to determine if you have the virus. At such times, be sure to ask any questions you have about how and why they are doing the test. It is important to be informed about your health.
` Antigens, antibodies, immune system, antigen tests, blood groups, HLA, infections
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