Eosinophils are a type of white blood cell. The immune system uses eosinophils and other white blood cells — or leukocytes — to ward off bacteria, viruses, parasites, and other possible causes of infection.

Eosinophils are a vital part of the immune system. They help fend off unwanted substances, known as pathogens. This process involves inflammation. Inflammation is a necessary part of the immune response, but it can sometimes be problematic.

For example, in the case of food allergies, the body mistakenly perceives certain ingredients as dangerous. As the immune system launches an attack, the numbers of eosinophils rise, and unhelpful inflammation can result, leading to swelling and other allergy symptoms.

Eosinophilia is when eosinophilia levels are high. They can rise when a person has an allergy, certain infections, or some types of leukemia.

Eosinopenia is when levels are low. This can affect people who use steroid medications, those with Cushing’s syndrome, and some people with acute inflammation, such as sepsis.

Here, learn about eosinophils, what they do, and what can happen if levels are too high or too low.

diagram eosinophilShare on Pinterest
Medical Illustration by Bailey Mariner

Eosinophils are known as effector cells. Effector cells are short-lived, activated cells that defend the body from bacteria, viruses, parasites, and other pathogens.

The body makes eosinophils in the bone marrow. From there, they travel to tissues throughout the body. They can be present in all body tissues, but the highest concentrations are in the respiratory and digestive systems.

They stay for 8–12 days, although their half-life — the time during which they are most effective — is 4.5–8 hours.

In most people, eosinophils represent around 0.5–1% of all white blood cells and rarely exceed 5%. If a person has high levels of eosinophils, doctors call this eosinophilia.

If blood tests detect eosinophilia, a doctor may recommend further tests to determine whether a person has a health condition such as leukemia, an infection, or an allergy.

During an allergic reaction, eosinophils may make up 3–5% of a person’s white blood count. Levels higher than 5% may indicate a parasite or eosinophilic leukemia.

Structure of eosinophils

Eosinophils have several key features:

  • a membrane that surrounds the cell
  • cytoplasm, a liquid inside the cells that surrounds the granules
  • around 200 microscopic granules that release substances such as cytotoxins, which kill unwanted cells
  • a nucleus with 2 lobes

The nucleus and secretion granules help doctors tell eosinophils apart from other white blood cell types.

Function

When the immune system detects an unwanted substance in a part of the body, such as bacteria, eosinophils will react to defeat them.

The role of eosinophils includes:

  • fending off bacteria and parasites
  • killing unwanted cells
  • participating in allergic reactions
  • playing a part in inflammatory responses

An eosinophil count measures the number of eosinophils in the blood.

Doctors can check as part of a complete blood count (CBC) or as a separate white blood count (WBC) test. A CBC will measure the various cells in the blood, including red blood cells and platelets.

A WBC will measure the number of:

According to the Cincinnati Center for Eosinophilic Disorders, the expected range for eosinophils is 0–450 eosinophils per cubic millimeter (mm3) of blood.

The presence of certain health conditions can affect a person’s eosinophil count. Eosinophil levels cannot diagnose a disease, but they can signal a problem that needs further investigation.

As well as testing the blood, a doctor may check for high eosinophil levels in a person’s body’s tissues, such as the nose or throat.

For example, a high concentration of eosinophils in the throat could indicate an allergic reaction to food or dust. It can indicate that eosinophils have gathered in that area to fight off a threat, real or perceived.

Eosinophils levels can rise, for example, when a person has:

  • an allergic reaction
  • an infectious disease
  • a disease that affects the immune system
  • leukemia, which affects the production of blood cells

High levels of eosinophils are called eosinophilia. This is not a disease but a sign that another health problem is present.

Doctors diagnose the following according to levels of eosinophils:

  • eosinopenia: very low levels of eosinophils, although there is no specific range
  • mild eosinophilia: 500–1,500 mm3
  • moderate eosinophilia: 150–5,000 mm3
  • hypereosinophilic syndrome: eosinophil counts are 1,500 mm3 or above on two occasions at least 1 month apart, or body tissues show high levels of eosinophils

Learn more about what an eosinophil count means.

A person can be healthy with zero eosinophils, so doctors do not usually consider eosinopenia a matter of concern.

However, some research suggests that eosinophil levels below 69 mm3 could indicate a health problem, such as sepsis and certain other infections. If a person has signs of infection and their eosinophil levels are low, this can help doctors identify the type of bacteria involved and how to treat it.

A 2020 study looked at blood test results for 208 patients who spent time in a hospital. Those with infections were more likely to have eosinophil levels below 69 mm3 than those without an infection. In 2021, other researchers reported a similar trend for people with COVID-19.

A 2017 study found that when people with Cushing’s syndrome had high cortisol levels in their blood and urine, their eosinophil levels were low. Conversely, lower levels of cortisol correlated with higher levels of eosinophils.

In people with Cushing’s syndrome, the body produces too much cortisol.

Having too many eosinophils can indicate several conditions.

A high level of eosinophils in the blood or body tissues is known as eosinophilia.

This can happen with various diseases and health conditions, such as:

  • parasitic, bacterial, fungal, or viral infections
  • myeloid leukemia
  • Hodgkin lymphoma
  • mastocystitis, when too many mast cells collect in the skin, bone marrow, liver, and other organs
  • cancerous tumors
  • eosinophilic granuloma, a benign tumor on the bone
  • allergies
  • rhinitis and other upper respiratory diseases
  • asthma and eosinophilic asthma
  • other lower respiratory diseases
  • immune-mediated diseases, such as systemic sclerosis and primary biliary cirrhosis
  • atopic dermatitis, an autoimmune disease
  • chronic spontaneous urticaria, where wheals, angioedema, or both develop on the skin
  • sensitivity to certain drugs, such as antibiotics
  • eosinophilic esophagitis, which affects the gullet
  • eosinophilic gastroenteritis and colitis
  • eosinophilic granulomatosis with polyangiitis (EGPA), previously known as Churg-Strauss Syndrome, which combines features of asthma with inflammation of small blood vessels and other tissue damage

What happens in eosinophilic diseases?

In some immune-mediated diseases, such as eosinophilic esophagitis, the eosinophils cause symptoms.

To ward off a perceived threat, such as a food or pollen allergy, a large number of eosinophils collect in the esophagus.

This causes inflammation, resulting in swelling, reflux, and difficulty swallowing.

Here are some answers to questions people often ask about eosinophils.

What causes eosinophil levels to rise?

High eosinophil levels can indicate that the body is responding to an infection or allergen. Specifically, it can be a sign of asthma, an allergic reaction, leukemia, and various types of infection.

What are eosinophils, and what do they do?

Eosinophils are a type of white blood cell. They play a key role in the immune system. They act to defeat pathogens, such as bacteria, viruses, fungi, and parasites. They also react to perceived threats, for example, during an allergic reaction. While essential for defending the body, their activity can cause inflammation. This can sometimes lead to further issues.

What is an eosinophil count?

A blood test will show a person’s eosinophil count. A low count is not usually a cause for concern, but a count of over 500 mm3 may indicate an underlying health problem. Sometimes, a doctor may take a tissue sample to test for eosinophils.

Eosinophils are a type of white blood cell. They are essential in the body’s immune response, as they contain proteins and other substances that can kill, destroy, or deactivate bacteria and other pathogens.

A low eosinophil count is not usually a cause for concern, but a high count may be a sign of a health condition, such as asthma, an infection, leukemia, or an autoimmune disease.