Eosinophils: Definition & Function

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  • 0:03 What Are Eosinophils?
  • 0:48 Functions
  • 1:38 Eosinophilic Abnormalities
  • 2:03 Lesson Summary
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Lesson Transcript
Instructor: Laszlo Vass
In this lesson, you'll learn about the structure and function of eosinophils, including where they are made and used in the body, their function(s) in the immune response, and how too many or too few of these cells can affect bodily function.

What Are Eosinophils?

Eosinophils are a type of white blood cell known as a granulocyte. They are characterized by a large, bi-lobed nucleus that resembles an old-style telephone receiver. Eosinophils are made in the red bone marrow and are rare in normal individuals, making up about two to four percent of all white blood cells. They range in size from about 10 to 15 micrometers. The cytoplasm of eosinophils contains granules. These granules are like small lysosomes and are filled with digestive enzymes. Eosinophils get their name from the dye eosin, which colors the granules brick red. These cells are mostly found in loose connective tissues in the body, including glands, reproductive organs, and the digestive tract.


The primary function of eosinophils is to respond to and destroy parasitic worms, such as pinworms, hookworms, tapeworms, and flukes. These worms are either ingested (often in raw fish), or they burrow into the skin and then invade the mucosae in the intestines or respiratory tract. Eosinophils release digestive enzymes from the granules in their cytoplasm onto the surface of the worms, thereby destroying them and digesting away their remains. They then play a role in phagocytosis and clean up some of the debris from the damage they caused.

Eosinophils are also involved in allergic responses and in the reactions involved in asthma. These latter functions are complex and they work in conjunction with other white blood cells, like basophils and mast cells, to accomplish these reactions.

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