What Is an Antigen?

Recognizing antigens is crucial in the function of the immune system

An antigen is a molecule that stimulates an immune response by activating lymphocytes, which are white blood cells that fight disease. Antigens may be present on invaders such as cancer cells, bacteria, viruses, parasites, fungi, and transplanted organs and tissues. Learn more about antigens and how the immune system interacts with them to protect you.

Antibodies respond to antigens on virus
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The Immune System

The human body relies on certain defenses to help keep sickness at bay. The defense system, known as the immune system, works in conjunction with many other processes to stomp out bodily threats on a cellular level. It does this by triggering a response that leads to the production of cells that will fight off infections.

There are two types of immunity at work within the body—innate and acquired.

Innate immunity is a type of protection against pathogens that’s nonspecific. It’s generally the first part of the immune system to respond to the appearance of an antigen, but It doesn’t have the ability to memorized certain threats and mount a specific defense if they show up again.

Acquired immunity is a little different in the way that it responds to the appearance of antigens. It is the part of immunity that works to identify the difference between threatening cells, targeting only outside pathogens. Acquired immunity is typically the process that gets activated when an antigen is present.

How It Works

The body needs to be able to recognize what belongs and what doesn’t, and antigens are recognizable by the immune system. This helps the body determine whether or not an immune response is needed by identifying the specific antigen.

Antigens bind to receptors on lymphocytes (a type of white blood cell). This then causes the multiplication of lymphocytes and triggers the immune response. The immune response can include producing specific antibodies against the antigen.

Types

Antigens can be divided into two main groups, both of which work differently to fight off infection in the body. These groups are known as foreign antigens and autoantigens.

Foreign Antigens

Otherwise known as heteroantigens, this type comes from outside of the body and are present on bacteria, viruses, snake venom, certain food proteins, and cells from other people. When it comes to lymphocyte response to foreign antigens, it’s likely that for them to do so the innate immune system must first be activated.

Autoantigens

Self-antigens are already present within the body with the immune system being able to clearly recognize them against other cells. These antigens don’t trigger an immune response in healthy individuals because the body knows they’re not harmful.

The Role of Antigens

As mentioned above, the antigen is the immune response initiator. It can either be bound by a specific antibody present in secretions or the blood, or by a B cell antigen receptor. The B cell antigen receptor is a transmembrane protein that is also a form of antibody.

Vaccines 

A vaccination is a medical injection that contains a weakened or dead version of the specific pathogen it is meant to prevent, or only part of the germ or toxin that would cause a disease. Vaccines are used to encourage an immune response within the body to create the specific antibodies needed to bind to that particular antigen.

When the immune system creates a specific antibody, such as an influenza antibody, when you do come into further contact with the virus your body is well-equipped to fight it off by using the previously created antibodies.

Once vaccinated, these specific antibodies are created for years following the initial entrance into the body. This can produce immunity to that particular strain due to the body’s ability to recognize the specific antigen if it happens to show up again.

In Viral Infection

In a viral infection such as the seasonal flu, the immune system develops a response by creating antibodies that can bind to the specific antigen. The process works in a similar way as it would with a vaccine, although the viral germs are much stronger in a live version.

The antigens that enter the body signal the immune response, thus causing the body to create antibodies for the specific strain of viral infection. These antibodies then utilize what is known as immunological memory.

The immunological memory is your immune system’s ability to ward off further illness from the same strain of disease using the antibodies it previously created in response to antigens.

The Role of Antibodies

Antibodies are created by cells within the immune system. They are produced to respond to specific antigens when they appear, to bind and eliminate the threatening pathogens from the body. They do this by neutralizing the threat it possesses or by alerting the proper part of the immune system to take over.

Significance

Antigens are important to the overall healthy function of the body because, without them, it’s unlikely people would make it through any foreign substance infection. Antigens are an important part of the immune response, and an immune response is required to keep the body free of any harmful substances.

If antigens aren’t present, the proper immune response wouldn’t be initiated and the bacteria or virus would be free to damage cells.

Antigens are also present on the surface of the cells of your body. These can play a significant role in the case of a transplant or transfusion, since the cells of another person may be recognized as foreign and trigger an immune reaction.

Testing Relevance

Tests for antigens and antibodies are the staple of clinical laboratories where your doctor sends your blood samples. These tests can help diagnose illnesses, prevent immune reactions, or check to see whether you have responded to a vaccine.

Antigen Test

Antigen tests are used to diagnose illnesses that are currently present in the body. Unlike antibodies which can tell whether a person has ever had a virus or other pathogen, antigen tests can only determine an ongoing infection. This is because the antigen disappears along with the pathogen it was bound to.

For example, in terms of COVID-19, antigen tests are likely the first line of discovery because they can determine whether or not a person is ill with the virus at the current time. This is important to help ward off the spread of the infection to other people.

Antibody Test

An antibody test works differently than the antigen test in the sense that it can be done long after the antigens have left the body. This specific test is used to determine whether or not an infection had ever occurred by singling out the antibodies that were created when the immune response took place.

As opposed to COVID-19 antigen testing, antibody testing can be done any time following the infection. It can be utilized to help isolate the viral information, seeing who has been previously exposed and developed an immune response. In developing a vaccine, researchers will look to see whether a vaccinated person developed antibodies.

Blood and Tissue Antigen Testing

Testing for different blood or tissue antigens is a very important aspect of blood transfusion or tissue or organ transplant.

In the case of blood transfusion, blood types must match A, B, and O antigens between donor and recipient. If not matched, the body has preformed antibodies that can immediately attack the unmatched red blood cells. The resulting transfusion reaction can be fatal.

Similarly, tissue typing such as for human leukocyte antigen (HLA) is commonly done before organ or tissue transplant. Matching these can help prevent organ or tissue rejection.

A Word From Verywell

Antigens can often be confused with antibodies, but the two hold very distinct positions when it comes to warding off pathogens that could lead to detrimental infection within the body. The antigen acts as more of an eliminator and antibody generator when it binds with certain immune cells.

Antigens may not be the main attraction when it comes to immunity, but they play a crucial role in the prevention and elimination of diseases.

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