What Are Eosinophils?

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Eosinophils are a type of white blood cell (WBC) and part of the immune system. They work with other WBCs and immune proteins to help the body fight infections, such as viruses, bacteria, fungi, and parasites.

These cells play a role in the innate immune system, which means that they react to infections rapidly and without specificity. Eosinophils are considered important for fighting parasites, which are infectious organisms that often have a complex life cycle.

Eosinophils are often involved in conditions that are associated with excessive inflammation, such as allergies, asthma, and autoimmune diseases. This article will discuss the function of eosinophils, how the body produces them, how they are counted and analyzed, and the meaning of a high or low eosinophil count.

Microscopic view of stained blood smear with eosinophil on left and segmented neutrophil on right surrounded by red blood cells

Ed Reschke / Getty Images

What Do Eosinophils Do? 

Eosinophils work with other WBCs and immune proteins to help destroy and remove infectious material from the body. They do this through several different mechanisms. One way is by signaling other WBCs, especially lymphocytes (B cells and T cells). 

Eosinophils are a type of granulocyte, which is a subset of WBCs that also includes neutrophils. In addition to signaling to other white blood cells, eosinophils also directly fight infections when they degranulate, which is a process by which they release their internal material.

Degranulation releases the following proteins that are toxic to infectious organisms:

  • Major basic protein (MBP)
  • Eosinophil cationic protein (ECP)
  • Eosinophil peroxidase (EPO)
  • Eosinophil-derived neurotoxin (EDN)

These proteins damage infectious organisms so they can’t continue to multiply and harm tissues in the body. 

Other Functions

Eosinophils are also involved in functions besides immunity, including metabolism, tissue remodeling and development, and regulating neurons (nerve cells), epithelial tissues, and the microbiome (the community of microbes in or on the body).

Where Do Eosinophils Develop? 

Eosinophils develop within the bone marrow (the spongy tissue inside of bones) which takes over a week. They then migrate to the tissues, where they survive three to five days. They reside in tissues throughout the body, where they can quickly become activated to help fight infection if necessary.

Eosinophil Count 

Your eosinophil count and concentration can be measured with certain tests. The most common way to measure eosinophils is with a complete blood count (CBC) with differential. This common blood test can measure the total WBC count, individual types of WBCs, red blood cells, and platelets.

According to the Centers for Disease Control and Prevention (CDC) eosinophilia (a high eosinophil count) may be defined as an eosinophil percentage higher than 5% or an absolute eosinophil count higher than 400 eosinophils per cubic milliliter.

A high or low eosinophil count may indicate certain medical conditions and disorders. For example, a high eosinophil count is a diagnostic feature of eosinophilic asthma, a specific type of asthma.

Bone Marrow Biopsy

Eosinophils can also be examined with a bone marrow biopsy. This is not a standard test. It’s typically used when there is concern about potential bone marrow disease, bone marrow cancer, or blood cell cancer. This test requires a procedure.

A CBC with differential or a bone marrow biopsy can be used to estimate the number and concentration of eosinophils.

Sometimes the blood cells are examined microscopically, which can help evaluate any changes in their structure or appearance. Microscopic examination of eosinophils is standard with a bone marrow biopsy, but it may also be done in a CBC with manual differential.

What Causes High Levels of Eosinophils? 

Certain conditions can cause eosinophilia. The different physical actions that eosinophils use to fight infection may cause unwanted symptoms when eosinophilia occurs. These symptoms may include swelling, skin redness, rash, runny or stuffy nose, and shortness of breath.

The symptoms may occur at the location where there are too many of these WBCs, or symptoms of eosinophilia can be systemic (affect the whole body).

Some of the causes of eosinophilia include:

  • Allergies
  • Eosinophilic asthma (inflamed airways with a high number of eosinophils)
  • Asthma
  • Eosinophilic gastrointestinal diseases (EGID), which includes eosinophilic esophagitis, eosinophilic gastroenteritis, and eosinophilic colitis
  • Eosinophilic pneumonia (an increase in eosinophils causing lung inflammation)
  • Autoimmune disorders, such as lupus and Crohn’s disease
  • Pancreatitis (inflammation of the pancreas)
  • Blood cell cancer
  • Churg-Strauss syndrome (autoimmune inflammation of the blood vessels)
  • Idiopathic hypereosinophilic syndrome (high eosinophils without an identified cause)

The symptoms of each of these conditions differ from each other, and they are usually noticeable. For example, asthma causes wheezing and shortness of breath, and blood cell cancer can cause lymph node swelling, bruising, and fatigue. 

A blood test may reveal eosinophilia. But sometimes, the increase in blood eosinophils is minimal, and it’s not consistently detected by blood tests.


High levels of eosinophils can be treated with corticosteroids, which also lowers other immune cells. Additionally, certain biologic treatments that target the IL-5 receptor or IL-4/IL-13 may disrupt eosinophil development to help lower the number of eosinophils and the effects of eosinophilia.

What Causes a Low Level of Eosinophils? 

Medications that suppress the immune system or cause damage to the bone marrow may lead to a low number of eosinophils. Not having enough eosinophils can make it difficult for the body to fight infection effectively, especially infections caused by parasites.

A low eosinophil count has also been associated with a short-term risk of cardiovascular disease, but the association weakens beyond six months.

Often, when eosinophils are low, other blood cells may also decrease in number. This occurs due to the same process that affects the eosinophil number. For example, bone marrow damage or chemotherapy (medication to treat cancer) can cause a decrease in most types of blood cells, including WBCs and red blood cells.

Sometimes a decrease in eosinophils might not cause symptoms.

The monoclonal antibody Fasenra (benralizumab) abolishes eosinophils from the body. This medication is directed to the interleukin-5 receptor to treat certain conditions associated with eosinophilia. At least one study showed that this loss of eosinophils might not have an adverse effect.


Eosinophils are one of the types of WBCs that are involved in the immune system. This type of WBC is specifically associated with helping fight parasitic infections. Also, some immune reactions such as allergies, asthma, and autoimmune diseases, are associated with increased eosinophil activity.

You can have certain tests to determine the approximate number and concentration of eosinophils in your blood and bone marrow. A standard CBC with differential can measure the approximate number of eosinophils in the blood.

A bone marrow biopsy can help your healthcare providers examine the number of developing eosinophils, as well as their appearance and other characteristics.

If you have any changes in your eosinophil count, you may have symptoms, and there are treatments to help alleviate the symptoms and the underlying condition.

7 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Hilvering B, Koenderman L. Quality over quantity; eosinophil activation status will deepen the insight into eosinophilic diseases. Respir Med. 2022:107094. doi:10.1016/j.rmed.2022.107094

  2. Upparahalli Venkateshaiah S, Manohar M, Kandikattu HK, Mishra A. Experimental modeling of eosinophil-associated diseases. Methods Mol Biol. 2021;2241:275-291. doi:10.1007/978-1-0716-1095-4_21

  3. Wechsler ME, Munitz A, Ackerman SJ, et al. Eosinophils in health and disease: a state-of-the-art review. Mayo Clin Proc. 2021;96(10):2694-2707. doi:10.1016/j.mayocp.2021.04.025

  4. Centers for Disease Control and Prevention. Complete blood count.

  5. Gleich GJ, Leiferman KM. Are eosinophils needed for normal health? Mayo Clin Proc. 2022;97(4):805. doi:10.1016/j.mayocp.2022.01.036

  6. Shah AD, Denaxas S, Nicholas O, Hingorani AD, Hemingway H. Low eosinophil and low lymphocyte counts and the incidence of 12 cardiovascular diseases: a CALIBER cohort study. Open Heart. 2016;3(2):e000477. doi:10.1136/openhrt-2016-000477

  7. AstraZeneca. How Fasenra works - mechanism.

By Heidi Moawad, MD
Heidi Moawad is a neurologist and expert in the field of brain health and neurological disorders. Dr. Moawad regularly writes and edits health and career content for medical books and publications.