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.

The body makes eosinophils in the bone marrow where they take 8 days to mature. They have an important role in the immune system, and healthy levels need to be maintained for proper functioning.

Fast facts on eosinophils

Here are some key points about eosinophils.

  • Eosinophils are a form of white blood cell.
  • High amounts can cause autoimmune conditions.
  • Treatment options for eosinophilia can include corticosteroids.

Eosinophils have several key features that help doctors tell them apart from other white blood cell types. These features are:

Image of an eosinophil.Share on Pinterest
Eosinophils are a type of white blood cell that fights bacteria and parasites.
  • a nucleus with 2 lobes
  • 200 microscopic granules inside the cells

Once they circulate in the blood, eosinophils do many things, including:

  • fend off bacteria and parasites
  • kill cells
  • participate in allergic reactions
  • play a part in inflammatory responses
  • "respond" 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 play a part in conditions such as food allergies and inflammation in the body's tissues.

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.

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 0-450 eosinophils per cubic millimeter of blood. Eosinophils aren't always present when a person is sick.

Eosinophils are 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. 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.

Higher-than-normal level of eosinophils can lead to a condition known as eosinophilia. When eosinophils are higher than 1,500, this is known as hypereosinophilic syndrome.

Low levels of eosinophils (eosinopenia)

As normal levels of eosinophils can be zero, 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. An example of this is drunkenness. Others are some medical conditions that cause the body to produce too many steroids. An example of this is an overproduction of cortisol, which can restrain the immune system.

High levels of eosinophils (eosinophilia)

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:

  • 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. 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.

Too many eosinophils cause a number of medical conditions. These conditions range from the occasionally annoying to the deadly. The following are examples of some but not all conditions that are caused by eosinophilia.

Eosinophilic pneumonia

Eosinophilic pneumonia occurs when a person has too many eosinophils in their lung tissue. The effects are:

  • difficulty breathing
  • muscle aching
  • coughing blood
  • coughing mucus
  • muscle aches
  • lower than normal oxygen in the blood
  • in rare cases, a person can experience respiratory failure

A person can have acute eosinophilic pneumonia, which causes a sudden, quick progression of pneumonia. It is unknown 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
  • parasitic infections

Eosinophilic esophagitis

Eosinophilic esophagitis (EoE) is an allergic reaction in the food pipe, 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 food pipe. This causes symptoms like:

  • stomach pain
  • difficulty swallowing (dysphagia)
  • nausea

Causes of EoE include:

  • food allergies, the most common cause
  • allergies to pollen
  • dust mites
  • animals
  • 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.

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Asthma is sometimes associated with eosinophilia.

The disease first causes symptoms such as:

  • asthma
  • growths in the nose

Eventually, it damages the body's nerves. This can cause a number of serious symptoms such as:

  • shooting pains
  • muscle wasting
  • severe tingling
  • trouble moving the hands and feet

Valley fever

Valley fever is a fungal infection also known as coccidioidomycosis. 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 of the U.S. (Arizona, California, Nevada, New Mexico, Texas, or Utah), or parts of Mexico, or Central or South America are most likely to get this condition, as this is where the fungus mostly grows.