Antigens vs. Pathogens

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  • 0:01 Pathogens
  • 1:45 Antigens
  • 2:32 Immune Response
  • 4:37 Lesson Summary
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Lesson Transcript
Instructor: Bridgett Payseur

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

Pathogens are small agents that can make you sick. Antigens are a part of the pathogen that your body recognizes. This lesson discusses how they differ and how your body protects against them.


You might have heard the words 'pathogen' and 'antigen' in your doctor's office, in news about the latest food poisoning outbreak, or in a commercial for new medications. But what exactly do these words mean, and what's the difference between the two?

To understand the difference between antigens and pathogens, it helps to define what both are. Pathogen is a fancy science word for germ. It's a microbe that can make you sick. This includes the usual bacteria and viruses and also some protozoans and fungi. Anything that's too small to be seen with the naked eye that can cause an infection is termed a pathogen.

Most of the pathogens you hear about are bacteria and viruses. Bacteria are very small, single-celled organisms. Strep throat is a sickness caused by the Streptococcus bacteria.

Viruses are even smaller than bacteria. Unlike bacteria, viruses cannot live on their own. They require a host cell to complete their life cycle. A virus takes over a host cell to make more copies of itself. The common cold, influenza, and chicken pox are all sicknesses caused by viruses.

Protozoans and fungi are less common pathogens than bacteria and viruses, but they can still be very dangerous. Both protozoans and fungi are larger and more complex than bacteria. Malaria is a potentially deadly infection of red blood cells that is caused by single-celled protozoans. Candidiasis, commonly called a yeast infection, is caused by an overgrowth of a fungus called Candida albicans.

While many different types of cells can be pathogens, the important thing to remember is that a pathogen is an invader trying to make the body sick.


If pathogens are the germs that make you sick - the invaders that attack your body - then what are antigens? Simply put, antigens are parts of the pathogen that tell your body it's being attacked. They're like warning banners the pathogens carry to alert your immune system to an attack. Antigens can be any type of molecule: protein, carbohydrate, nucleic acid, or even fat.

Why would a pathogen even bother having these warning signals? Well, often antigens are important components that the pathogen needs to infect the body. Common antigens are proteins on a virus, such as influenza, that help the virus enter the cell. The pathogen's need for a protein to infect the host is in constant combat against the body's ability to recognize the antigen and stop the infection.

Immune Response

The body uses antigens to locate and destroy pathogens. Two types of white blood cells can recognize antigens and begin a coordinated defense strategy: B cells and T cells.

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