Basophils are white blood cells from the bone marrow that play a role in keeping the immune system functioning correctly.

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

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

What are basophils?

Basophils are a type of white blood cell, which are vital components of the immune system.

The body makes different types of white blood cell, which are vital components of the immune system.

White blood cells help keep the body healthy by fighting off invading germs, such as bacteria, viruses, and fungi.

Basophils are a type of white blood cell called a granulocyte. There are other forms of granulocyte, such as neutrophils and eosinophils.

Granulocyte cells contain granules, which they use to secrete important substances.

The granules inside basophils contain heparin, histamine, and other molecules that play a role in inflammation.


Basophils are necessary for the immune system’s natural response to invaders, such as infectious germs.

The body’s response to allergens also involves basophils. When a potentially harmful allergen enters the body, the immune system responds by trying to isolate and eliminate the allergen.

When responding to an allergen, basophils that sustain damage will release histamine, which is partially responsible for inflammation during an allergic reaction.

Additionally, basophils play an integral role in preventing blood clotting. The heparin inside the cells is a form of natural blood thinner that helps keep the blood flowing through the body.


Doctors believe that the role of basophils in the body is reactionary, meaning that their number will generally only rise or fall due to an invader or underlying chronic issue.

This characteristic allows doctors to use basophil tests to help them identify underlying conditions and severe allergic reactions.

Doctors can use a complete blood count (CBC) to check a person’s basophil levels. A basophil count that is higher or lower than the normal range may prompt them to order additional tests.

A white blood cell count (WBC) test may be necessary to find the absolute basophil count in some cases. 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.

During the BAT, medical professionals in a laboratory apply potential allergens to a sample of the person’s blood. If the person has an allergy, the basophils in their blood sample will activate specific molecules.

A 2016 study found that the BAT is highly accurate in confirming food allergies. It may also be helpful for monitoring the immune system’s response to food allergens.

The BAT is a low-risk test compared with the oral food challenge test, which has the potential to trigger a dangerous allergic reaction.

Normal range

Although they have an essential function in the immune system, basophils only make up a small percentage of the total number of white blood cells. In a normal test result, they may contribute to less than 0.5 percent of the total white blood cell count.

Causes of high levels

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

Basophils rheumatoid arthritis
High levels of basophils may indicate rheumatoid arthritis.

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

Immune reactions or autoimmune conditions that cause chronic inflammation include:


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 very easily.

Myeloproliferative disorders

Myeloproliferative disorders affect white blood cells and may also cause very 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 deformed or 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.


In very rare cases, high basophil levels may indicate certain types of blood cancer, including leukemia and lymphoma.

Causes of low levels

Basopenia is the medical term for abnormally low basophil levels.

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

Conditions that can cause low levels of basophils include:


In people with hyperthyroidism, the thyroid gland overproduces thyroid hormones, causing bodily functions to speed up.

Hyperthyroidism may cause noticeable signs and symptoms, such as:

Allergic reactions

Basophils allergy
Having an allergic reaction may cause low levels of basophils.

Low levels of basophils may be due to the body reacting to an allergen, causing the basophils to release their histamine. Other signs 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. Signs of anaphylaxis include:

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

Anaphylaxis can be life-threatening. Anyone having an anaphylactic reaction should seek emergency medical attention.


Basophils are instrumental to immune system function, so low levels may also signal that the body is fighting an infection.

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


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.