Eosinophils: All You Need to Know

The immune system uses white blood cells to fight off bacteria, viruses, and other things that could cause infection. But it doesn't just use one white blood cell type to do it. Instead, there are several types of immune system cells. These include eosinophils.

Doctors named eosinophils because a special stain known as eosin turns the proteins in eosinophils pink. The "-phils" portion of the word eosinophils stands for "loving." So eosinophils are defined as eosin-loving.

Contents of this article:

What is an eosinophil?
What is an eosinophil count?
What do low or high eosinophil levels mean?
What conditions affect eosinophil levels?
What is an eosinophil?
Under a microscope, eosinophils have several key features that help doctors tell them apart from other white blood cell types:

Image of an eosinophil.
Eosinophils are a type of white blood cell that fights bacteria and parasites.
Have a nucleus with 2 lobes
Have 200 microscopic granules inside the cells
The body makes eosinophils in the bone marrow, where they take 8 days to mature. Once they circulate in the blood, doctors think eosinophils have many functions. These include:

Fending off bacteria and parasites
Killing cells
Participating in allergic reactions
Playing a part in inflammatory responses
"Responding" to areas of inflammation
Although eosinophils are part of the immune system, some of their responses aren't always healthy for the body. Sometimes they may play a part in conditions such as food allergies and inflammation in the body's tissues.

When eosinophils move to the tissues, they release poisons that are designed to kill foreign substances. However, the poisons can sometimes cause damage to tissues.

What is an eosinophil count?
An eosinophil count is a measure of the amount of eosinophils in the blood. A doctor can order a blood test known as a white blood cell count with differential.

Infographic of white blood cell types.
Eosinophils are just one kind of white blood cell.
The "differential" means that the lab will test not only how many white blood cells there are in the body, but also how many of each kind of white blood cell there are.

The result will measure the number of:

Eosinophils
Basophils
Lymphocytes
Monocytes
Neutrophils
A doctor may also order this test as a complete blood cell count with differential. This measures white blood cells as well as red blood cells and other parts of the blood.

The normal value for eosinophils may vary from lab to lab. Typically, a lab will include "reference ranges" that give the average results for that measurement.

According to the Cincinnati Center for Eosinophilic Disorders, the normal range for eosinophils is 0 to 450 eosinophils per cubic millimeter of blood. Eosinophils aren't always present when a person is sick. They're also found in the intestines, thymus, spleen, lymph nodes, ovaries, and uterus.

What do low or high eosinophil levels mean?
When compared to other types of white blood cells in the blood, eosinophils usually make up a small percentage.

According to the Mayo Clinic, a blood count of more than 500 eosinophils per microliter of blood is considered a higher-than-normal level of eosinophils. This is known as eosinophilia. When eosinophils are higher than 1,500, this is known as hypereosinophilic syndrome.

Low levels of eosinophils
Because normal levels of eosinophils can be 0, a low level of eosinophils isn't usually considered a medical problem after one test.

However, there are some conditions that can cause a low level of eosinophils, which is known as eosinopenia. This includes drunkenness and medical conditions that cause the body to produce too many steroids.

An example is an overproduction of cortisol, which can restrain the immune system.

High levels of eosinophils
A level of eosinophils between 500 and 1,500 per microliter of blood is known as eosinophilia. There are a number of causes of eosinophilia. Examples include:

A woman using an asthma inhaler.
Asthma can lead to high levels of eosinophils in the blood.
Allergies
Asthma
Abnormal blood cells known as hypereosinophilic myeloid neoplasms
Inflammatory conditions, such as celiac disease or inflammatory bowel disease
Inflammatory skin conditions, such as dermatitis or eczema
Cancerous growths including Hodgkin's disease
Parasitic infections
Reactions to medications
In addition to having high levels of eosinophils in the blood, it's also possible to have high levels of eosinophils in the body's tissues. While this occurs, a person's blood eosinophil levels may be normal. A doctor can test a tissue sample taken from a person to find out if eosinophil levels are too high. They can also test mucus from the nose.

Because an underlying condition causes eosinophilia, having high levels of eosinophils can cause varying symptoms in a person. A doctor will consider the underlying condition when treating eosinophilia.

What conditions affect eosinophil levels?
Too many eosinophils can cause a number of medical conditions. These conditions range from an occasional annoyance to deadly. The following are examples of some (but not all) conditions that cause eosinophilia.

Eosinophilic pneumonia
Eosinophilic pneumonia occurs when a person has too many eosinophils in their lung tissue. The results are difficulty breathing, muscle aching, coughing up blood or mucus, muscle aches, and lower than normal oxygen in the blood. Some people can even experience respiratory failure.

A person can have acute eosinophilic pneumonia, which causes a sudden, quick progression of pneumonia. Doctors don't know what causes this condition.

Another form, known as chronic eosinophilic pneumonia, results in a more lingering illness. Causes include blood cancers, fungal infections, autoimmune diseases, and parasitic infections.

Eosinophilic esophagitis
Eosinophilic esophagitis (EoE) is an allergic reaction in the esophagus, the thin tube that carries food from the mouth to the stomach.

An allergen triggers an immune system response that brings too many eosinophils to the esophagus. This causes symptoms like stomach pain, difficulty swallowing, and nausea.

Food allergies are the most common cause of EoE. Other causes include allergies to pollen, dust mites, animals, and molds.

Eosinophilic granulomatosis with polyangiitis
Eosinophilic granulomatosis with polyangiitis (EGPA) is a form of vasculitis. This causes inflammation in the blood vessels. The disease used to be known as Churg-Strauss Syndrome.

The disease first causes symptoms such as asthma and growths in the nose. Eventually, the condition damages the body's nerves. This can cause a number of serious symptoms, including shooting pains, muscle wasting, severe tingling, and trouble moving the hands and feet.

Valley fever
Also known as coccidioidomycosis, Valley fever is a fungal infection. When people breathe in the spores of the fungus Cocciodioides, the results can be a flu-like infection.

Symptoms of the condition include:

Coughing
Fever
Night sweats
Shortness of breath
Rash on the upper body or legs
Headache
People in the Southwest are most likely to get this condition, as this is where the fungus mostly grows.