Basophils are white blood cells that the bone marrow makes. They play a role in a healthy immune system. Having more or less basophils than usual may be a sign of inflammation, hyperthyroidism, or another health condition.

Doctors may order basophil level tests to help diagnose certain health concerns. If basophil levels are low, it may be a sign of an allergic reaction or another condition. High basophil levels may indicate an autoimmune disease or one of several types of blood disorders.

In this article, learn more about the function of basophils and what abnormal basophil levels mean.

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White blood cells are vital components of the immune system. The bone marrow makes white blood cells, and they help keep the body healthy by clearing out invading germs, such as bacteria, viruses, and fungi.

There are different types of white blood cells, including basophils, neutrophils, and eosinophils.

When scientists look at white blood cells under a microscope, they often use a combination of substances for staining: hematoxylin and eosin. Hematoxylin is alkaline and makes basophils appear blue under the microscope. Eosinophils show up as red when they come into contact with eosin, which is acidic.

Basophils are types of white blood cells called granulocytes. Granulocytes contain granules, which release important substances that influence the behavior of other cells and other biologic substances.

The granules inside basophils contain heparin, histamine, and other molecules that play a role in inflammation. Basophils have short lives, between a few hours and a few days.

The immune system uses basophils as part of its natural response to invaders, such as bacteria and viruses.

Allergic responses to allergens also involve basophils. When a potentially harmful allergen enters the body, the immune system responds by trying to isolate and eliminate the allergen.

When they respond to an allergen, some basophils become damaged. A basophil binding to an allergen disrupts the basophil’s cell membrane.

Additionally, basophils play an integral role in preventing circulating blood from unwanted clotting.

Basophils are reactionary, meaning their number rises or falls when they sense an invader or there is an underlying condition. This means doctors can use basophil tests to identify underlying conditions and severe allergic reactions.

Doctors use a complete blood count to check a person’s basophil levels. A basophil count that is higher or lower than usual may prompt them to order additional tests.

A white blood cell count test may be necessary to find the absolute basophil count. This test can help doctors get a better picture of the range of basophils in the blood.

Doctors may also order a specific test called a basophil activation test (BAT) to check for particular allergens.

The BAT involves testing for allergies by adding potential allergens to a blood sample. If the person has an allergy, the basophils in their blood activate specific molecules.

BAT is an accurate way to confirm food allergies. It may also help monitor the immune system’s response to food allergens.

The BAT is a low risk test compared with the oral food challenge test, which can potentially trigger a dangerous allergic reaction.

What does a high white blood cell count mean?

Basophils only make up a small percentage of the total number of white blood cells, even though they have an essential function in the immune system. Usually, they contribute to 0–1% of the total white blood cell count.

Blood tests may reveal basophil levels that are too high. The medical term for this is “basophilia,” and there are several possible causes:

Autoimmune inflammation

High levels of basophils may indicate chronic inflammation in the body.

Immune reactions or autoimmune conditions that cause chronic inflammation include:

  • rheumatoid arthritis
  • lupus
  • inflammatory bowel diseases (IBD), such as Crohn’s disease or ulcerative colitis
  • diabetes
  • allergies and asthma

Inflammation due to an infection

People with high basophil levels may have an infection causing inflammation, such as tuberculosis (TB) or influenza.

A doctor may recommend medications or rest until the infection clears, after which they will order blood tests to get more accurate results.

Allergic reactions

Low levels of basophils may be due to the body reacting to an allergen, causing the basophils to release their histamine. Other symptoms of an allergic reaction include:

  • puffy, red eyes
  • a runny or stuffy nose
  • excess mucus
  • hives

Severe allergic reactions may cause a potentially life threatening situation called anaphylaxis.

Symptoms of anaphylaxis include:

  • swelling in the face, throat, or mouth
  • breathing difficulties
  • experiencing lightheadedness
  • wheezing

Anaphylaxis can be life threatening. Anyone having an anaphylactic reaction must get emergency medical attention.

Anaphylaxis: Symptoms and what to do

Anaphylaxis is a severe allergic reaction that can be life threatening. The symptoms develop suddenly and include:

  • hives
  • swelling of the face or mouth
  • wheezing
  • fast, shallow breathing
  • a fast heart rate
  • clammy skin
  • anxiety or confusion
  • dizziness
  • vomiting
  • blue or white lips
  • fainting or loss of consciousness

If someone has these symptoms:

  1. Check whether they are carrying an epinephrine pen. If they are, follow the instructions on the side of the pen to use it.
  2. Dial 911 or the number of the nearest emergency department.
  3. Lay the person down from a standing position. If they have vomited, turn them onto their side.
  4. Stay with them until the emergency services arrive.

Some people may need more than one epinephrine injection. If the symptoms do not improve in 5–15 minutes, or they come back, use a second pen if the person has one.

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High basophil levels may also be a sign of low thyroid function, or hypothyroidism. This condition occurs when the body does not produce enough thyroid hormones, which may cause some bodily functions to slow down.

Hypothyroidism can cause a variety of symptoms, including:

Some people with hypothyroidism may also notice changes in their hair or skin. The skin might become dry or rough, while the hair may turn coarse and brittle and break easily.

Myeloproliferative disorders

Myeloproliferative disorders affect white blood cells and may also cause high basophil levels.

Myeloproliferative disorders include:

  • Myelofibrosis: In people with this condition, fibrous tissue begins to replace the cells that make blood in the bone marrow. This disruption may lead to misshapen red blood cells and anemia.
  • Essential thrombocythemia: This condition causes the body to make too many platelets, leading to excessive blood clotting. It may also lead to circulation and nerve problems.
  • Polycythemia vera: This is a blood condition that causes the bone marrow to overproduce red blood cells.


High basophil levels may indicate certain types of blood cancers, including leukemia and lymphoma. It can also be a sign of a solid tumor.

“Basopenia” is the medical term for low or reduced basophil levels. Basophil levels are usually low. However, if records show a person has lower levels than in previous tests, this can be a sign of a health concern.

Causes of basopenia include the following:

  • acute infection
  • hyperthyroidism, which is when the body produces too much thyroid hormone
  • stress reactions, for example during pregnancy or a myocardial infarction
  • long-term steroid therapy, chemotherapy, or radiation therapy
  • inherited genetic features that lead to low levels of basophils

When a basophil releases its granules in response to an invader or inflammation, it becomes empty. An empty basophil does not show up on blood tests, and the test may show a lower number of these cells.

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Basophils measure 12–15 micrometers (μm) in diameter and have double-lobed or s-shaped nuclei.

They contain small particles called granules that measure around 0.5 μm in diameter. When a scientist examines a blood smear slide under the microscope, these granules stain blue and purple using diagnostic stains and almost obscure the underlying nucleus.

A doctor may recommend further tests or treatment if a person’s blood test indicates unusually high basophil levels.

Treatment depends on the underlying cause:

  • Autoimmune inflammation: Doctors may prescribe medications, such as biologic therapy for IBD, for autoimmune inflammation.
  • Infections: If tests detect a bacterial infection, such as TB, a doctor will prescribe antibiotics.
  • Hypothyroidism: Treatment for this involves synthetic thyroid hormone replacement therapy.
  • Myeloproliferative disorders: These disorders may require chemotherapy, immunotherapy, radiation therapy, a stem cell transplant, or steroids as treatment.
  • Cancer: Cancer treatment may involve chemotherapy, radiation therapy, immunotherapy, or targeted therapy.

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

What does it mean if basophil levels are high?

High basophil levels can indicate a health concern, such as an autoimmune disease or an allergic reaction. They can also indicate some types of blood cancers, like leukemia, or a solid tumor.

What does it mean if basophil levels are low?

Low basophil levels, or basopenia, are normal and not usually a cause for concern. However, if medical records show that levels are too low, a doctor may consider the possibility of a health condition, such as an infection, a stress reaction, or if there are adverse effects to treatment, such as chemotherapy.

What are basophils, and what do they do?

Basophils are types of white blood cells. They work with the immune system to help protect the body from pathogens, such as bacteria and viruses.

Basophils make up a small percentage of white blood cells, but they play an essential role in the immune system. Basophil levels that are too high or too low may be a sign of an underlying condition.

There are many possible underlying causes of abnormal basophil levels. Once a doctor determines the reason for the high or low levels, they can advise on possible treatment options.