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Endogenous Antigen: Definition, Example & Processing

Instructor: Bridgett Payseur

Bridgett has a PhD in microbiology and immunology and teaches college biology.

An endogenous antigen comes from the body's cells. This lesson will discuss how cells alert the immune system to virus infection or cancer using endogenous antigens.

Endogenous Antigens

Let's pretend your community has been invaded by zombies. You and everyone else in the village bands together to fight them. You can tell a zombie by their walk and glowing red eyes. However, what if some of the zombies infected a few members of your village? Now how will you know who to attack? Eventually the infected person will show the red eyes and walk and give themselves away.

Oddly enough, your cells can work the same way. A virus or harmful bacteria enters your body, carrying with it little markers on its surface called antigens that the body recognizes. The body's cells then attack the foreign invaders.

However, sometimes a virus will invade an individual cell of your body's, and that cell is now a force for the enemy. The cell can do one last thing to help the body recognize it - show an endogenous antigen.

An endogenous antigen is a marker on your own cells that starts an immune response. At first glance the term is a bit intimidating. To make it less scary, it helps to break down the parts of the words. 'Endo' means inside, and in this case is referring to the inside of your own body or cells. 'Genous' means originating from. This antigen originates from inside your own cells.

Sources of Endogenous Antigens

Why would your body want to attack something from your own cells? Keep in mind that your body is not made up only of you. It is covered with and inhabited by trillions of bacteria, viruses, and other tiny living things called microbes. Most of these are harmless (actually, they are very helpful). However, sometimes the microbes in you are pathogens, meaning they can cause disease.

All viruses, including HIV, influenza, and measles virus, cause disease by infecting cells. A virus enters a cell, uses the cell's machinery to replicate itself, and then leaves to infect new cells. This infection cycle means that the host cell is now producing viral antigens.

Image showing virus infecting and replicating within a cell
Virus Life Cycle

Another time a cell might produce an antigen to start the immune response is when the cell becomes cancerous. Cancer happens when cells replicate excessively. Cancerous cells may produce different proteins that can be recognized as danger signals. This gives the body a chance to stop the cancer from spreading. Unfortunately, because cancer cells replicate and mutate so rapidly, the body is not always able to fight cancer on its own.

Immune Response

How do endogenous antigens start the immune response? By definition, an endogenous antigen is inside a cell, so how can it even be seen by immune cells?

A cell infected by a virus can actually call for help, in a way. Kind of like if a person bitten by a zombie warned their fellow villagers that they were about to become a zombie. The cell will need to process the antigen, so it can be used to start an immune response.

Let's look at a cell infected by influenza virus for an example. First, the virus will have the cell make some viral proteins. The cell will then break down some of the influenza proteins into short segments, and present these protein pieces on its surface. The viral protein pieces can then be recognized by cytotoxic T cells which will kill the cell. These cells arealso sometimes called cytolytic T cells, which get their names from their ability to kill, or lyse, cells (cyto=cell).

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