What Is an Antigen?

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

The Immune System

The human body relies on certain defenses to help keep sickness at bay. The immune system triggers a response that produces cells and proteins to fight off infections.

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

• Innate immunity is a type of nonspecific protection against pathogens. It responds quickly to a pathogen, but It doesn’t have the ability to remember individual threats and mount a specifically targeted defense if they show up again.
• Acquired immunity is the part of immunity that works to identify the difference between individual types of threats. Acquired immunity works more slowly than innate immunity, but it remembers the antigen and responds to it quickly and in a targeted manner if you are exposed again.

How It Works

The body needs to be able to recognize what belongs and what doesn’t, and antigens are an important part of that process. When the body identifies an antigen, it will initiate an immune response.

When receptors on white blood cells bind to antigens, this triggers white blood cell multiplication and starts the immune response.

Types

Antigens can be divided into two main groups—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.

Autoantigens

Self-antigens are already present within the body and shouldn't trigger an immune response in healthy individuals because the body should know they’re not harmful. However, sometimes the body erroneously acts against them—leading to autoimmune inflammation.

The Role of Antigens

Antigens are immune response initiators. They can be bound by white blood cells, including leukocytes, which are the cells of the adaptive immune system.

Leukocytes include B cells and T cells. B cells make antibodies that can also bind to antigens. After an antigen gets bound to a B cell receptor, antibodies are produced.

Vaccines 

A vaccination is a medical injection or pill that contains a protein or weakened or dead version of a pathogen. Vaccines are used to create an immune response within the body against a particular antigen.

When the immune system creates a specific antibody, such as an influenza antibody, this makes your body ready and well-equipped to fight off the influenza virus if exposed later by using the previously created antibodies.

Once you are vaccinated, your antibodies should remain ready to fight the infection for years.

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 infectious viral germs are much stronger.

The antigens on the infectious virus signal an immune response, causing the body to create antibodies for the specific strain of viral infection. These antibodies then utilize what is known as immunological memory to help you fight the infection if you are exposed again.

The Role of Antibodies

Antibodies are created by cells within the immune system. They bind to antigens and promote the elimination of threatening pathogens from the body. They neutralize the threat by alerting other parts of the immune system to take over.

Significance

Antigens are an important part of the immune response because they help your body recognize harmful threats to get rid of them.

Testing Relevance

Tests for antigens and antibodies can be done with 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.

For example, in terms of COVID-19, antigen tests 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 after the antigens have left the body. This 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 after the infection resolves. 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 the blood donor and recipient's antigens don't match, this signals that their bodies have antibodies that can immediately attack unmatched donor 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 to 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 an antibody generator and it gets eliminated (along with the infectious agent) by the body's immune system.

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.