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What Are Eosinophils? - Definition & Function

Lesson Transcript
Instructor: Lynee Carter
Eosinophils are white blood cells that protect the body from germs, specifically bacteria and parasites. These white blood cells engulf, excrete, and exhibit to defend the body from illness and allergic reactions. Learn more about the critical functions of eosinophils and how they protect the human body. Updated: 10/13/2021

Definition of Eosinophil

Your immune system has white blood cells that help detect and defend your body from germs and other foreign matter that can make you sick. One of these types of white blood cells is called an eosinophil. Eosinophils help protect your body from harmful bacteria, as well as from parasites that can steal important nutrients from your body.

Eosinophils are formed in the bone marrow, then released into the blood. They are also found in the tissues of the esophagus, intestines, stomach, heart, lungs, and skin. Being at these locations makes them closer to the sites where germs try to enter the body and thus better prepared to destroy them.

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  • 0:00 Definition of Eosinophil
  • 0:35 How Eosinophils Function
  • 0:55 Engulf
  • 1:20 Excrete
  • 2:20 Exhibit
  • 2:45 Lesson Summary
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How Eosinophils Function

Eosinophils function in several ways. You can remember how eosinophils function if you remember that they use the 'three Es' to fight foreign invaders: engulf, excrete, and exhibit.


Let's start with 'engulf.' As soon as a parasite enters the body, a chemical signal is sent out that the eosinophils recognize and tells them exactly where the intruders are located. When arriving to the site, one task they can perform is to completely engulf parasites. This is like you swallowing a large amount of food without chewing! It's definitely an effective way eosinophils can defeat these germs.


There are some parasites too big for the eosinophils to engulf. In these cases, they have to use a different method of attack - this is where 'excrete' comes in. They first attach themselves directly to the parasites. Then they excrete powerful enzymes that cause the invader to break down into smaller pieces making it easier to be engulfed. These enzymes can also disrupt how the invader functions, which eventually leads to its death.

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