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Eosinophils

What Are They, What Do They Do, and More

Author: Ashley Mauldin, MSN, APRN, FNP-BC

Editors: Alyssa Haag, Józia McGowan, DO

Illustrator: Jillian Dunbar

Copyeditor: Sadia Zaman, MBBS, BSc

What are eosinophils?

Eosinophils are a type of white blood cell (i.e., leukocytes) that are secreted in response to allergic reactions, skin conditions, parasitic and fungal infections, and autoimmune diseases, as well as certain cancers and bone marrow disorders. Eosinophils are specifically granulocytic leukocytes produced in the bone marrow, and can usually be found in the connective tissues, especially in the thymus, gastrointestinal tract, spleen, lymph nodes, ovaries, and uterus. 

What do eosinophils do?

Eosinophils play a role in the immune system by helping fight infections and increasing inflammation in the body. In the typical day-to-day functioning of the body, the absolute eosinophil count (i.e., a type of blood test that tells you the number of eosinophils circulating in the body) can be found in relatively low numbers, usually around 1-4% of the total white blood cell count in circulation. When the immune system is activated, the number of eosinophils released increases, and the eosinophils flock to the site of an infection to help combat the infectious agent or help incite an inflammatory reaction. 

When activated, eosinophils release highly toxic proteins and free radicals to help fight microorganisms and parasitic infections. These same proteins and radicals can also cause tissue damage when eosinophils are activated during allergic reactions. Eosinophil activation also causes the production of certain chemical mediators like prostaglandins, cytokines, and leukotrienes, all of which also assist in boosting the inflammatory response. 

What causes eosinophils to be low?

A low level of eosinophils can occur with certain conditions, like Cushing syndrome; treatment with certain medications, like corticosteroids; and severe bloodstream infections (i.e., bacteremia). 

What causes eosinophils to be high?

High levels of eosinophils, also known as eosinophilia, are usually a result of allergic reactions, including asthma, allergic rhinitis, and dermatitis; parasitic infections; and certain cancers, like Hodgkin lymphoma and leukemia. 

Sometimes, when there are high levels of eosinophils in the body, the eosinophils can move outside the bloodstream and accumulate on other organs and tissues. This can cause eosinophilic disorders and symptoms may occur depending on where the eosinophils accumulate. Some examples of eosinophilic disorders include eosinophilic cardiomyopathy, which can manifest with chest pain and shortness of breath; eosinophilic colitis, which can present as diarrhea; eosinophilic esophagitis, which involves difficulty swallowing; and eosinophilic pneumonia, which can manifest as cough and shortness of breath. 

In rare cases, eosinophils can increase to alarming levels and the individual can develop hypereosinophilic syndrome. This syndrome is diagnosed when, for a period of at least 6 months, blood tests reveal persistently high levels of eosinophils. 

Finally, eosinophilia can also be caused by a rare condition called eosinophilic granulomatosis with polyangiitis. This condition is characterized by high levels of eosinophils along with asthma, and inflammation of the blood vessels, also known as vasculitis.  

What are the most important facts to know about eosinophils?

Eosinophils are a type of white blood cell that play a role in the immune system by helping fight infections and boost inflammation in the body. When the immune system is activated, the number of eosinophils released increases, and the eosinophils flock to the site of infection to help combat the infectious agent or help incite an inflammatory reaction. High levels of eosinophils, also known as eosinophilia, are usually caused by allergic reactions, including asthma, allergic rhinitis, and dermatitis; parasitic infections; and certain cancers, like Hodgkin lymphoma and leukemia. Sometimes, when there are high levels of eosinophils, the eosinophils can migrate outside the bloodstream and accumulate in other organs and tissues causing eosinophilic disorders. In rare cases, eosinophils can increase to alarming levels and the individual can develop hypereosinophilic syndrome. A low level of eosinophils can occur with certain conditions like Cushing syndrome; treatment with certain medications, like corticosteroids; and severe bloodstream infections, also known as sepsis. 

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