Granulocytes are white blood cells (WBCs) that have small granules or particles. These granules contain numerous proteins that are responsible for helping the immune system fight off bacteria and viruses.

Granulocytosis occurs when there are too many granulocytes in the blood.

An abnormally high WBC count usually indicates an infection or disease. An increase in the number of granulocytes occurs in response to infections, autoimmune diseases, and blood cell cancers.

Granulocytosis is closely related to chronic myeloid leukemia (CML) and other bone marrow disorders.

Granulocytes form and mature in the bone marrow — the spongy tissue found inside many of your bones.

Bone marrow contains stem cells, which eventually develop into different types of blood cells, including granulocytes.

When granulocytes leave the bone marrow, they circulate through the bloodstream and respond to signals from the immune system. Their role is to attack foreign substances that cause inflammation or infection.

Neutrophils, eosinophils, and basophils are the key types of granulocytes.

Neutrophils typically account for 40 to 70 percent of all WBCs. Up to 4 percent of your WBCs may be eosinophils, while basophils account for under 2 percent.

The presence of granulocytes in the bloodstream is normal. These WBCs are part of your immune system and help defend your body against harmful bacteria and viruses.

However, a high number of granulocytes in the blood isn’t normal and usually indicates a health problem.

Bone marrow disorders are a major cause of granulocytosis. The following bone marrow disorders can cause granulocytosis:

Granulocytosis can also be seen in combination with:

Granulocytosis is the main feature of CML, a rare blood cell cancer that begins in the bone marrow.

CML causes a buildup of underdeveloped granulocytes in the bone marrow and bloodstream.

Normally, bone marrow produces immature stem cells in a controlled way. These cells then mature and turn into RBCs, WBCs, or platelets. Your RBCs carry oxygen and nutrients, your WBCs help fight infection and inflammation, and your platelets enable the blood to clot.

In people with CML, this process doesn’t work correctly. Immature granulocytes and other WBCs begin to form and multiply uncontrollably, crowding out all the other types of necessary blood cells.

People with CML may experience the following symptoms:

CML is most common among older adults, but it can occur in people of any age. It also affects men more than women.

People who have been exposed to radiation, such as radiation therapy for the treatment of cancer, have a higher risk of developing CML.

Granulocytosis is normally diagnosed with a physical examination and a complete blood count (CBC). The CBC is a test that measures the amount of RBCs, WBCs, and platelets in your blood.

Abnormal numbers of these cells can indicate that you have an underlying medical condition.

The CBC involves giving a sample of blood. You’ll have blood drawn from a vein in your arm. The blood sample will then be sent to a lab for analysis. As with any blood draw, there’s a small chance of discomfort, bleeding, or infection.

Reference ranges will vary by lab. However, you typically have the following amount of granulocytes in your blood, according to Lab Tests Online:

  • Neutrophils: 1,800–7,800 cells per microliter (mcl) of blood
  • Eosinophils: 0–450/mcl of blood
  • Basophils: 0–200/mcl of blood

Amounts that are higher than this may indicate granulocytosis.

Granulocytosis is a symptom of other conditions. It’s not considered a separate disease, and it usually isn’t treated directly.

Instead, treatment addresses the underlying condition causing granulocytosis. Treating any underlying conditions should also reduce the number of granulocytes in your blood.

Your treatment will depend on the condition causing your granulocytosis.

If your condition is related to cancer, your treatment may include the following:

  • Bone marrow transplant. During a bone marrow transplant, your bone marrow will be removed and replaced with healthy stem cells. These stem cells may come from your body or from a donor’s body.
  • Chemotherapy. Chemotherapy is an aggressive form of chemical drug therapy that helps destroy cancerous cells in the body.
  • Radiation therapy. This treatment uses high-energy radiation to shrink tumors and kill cancerous cells.
  • Splenectomy. An enlarged spleen is a common complication of CML. Surgery to remove the spleen may be recommended.

Some conditions respond well to medications, and other conditions can be treated with blood transfusions. Your doctor will determine the best treatment plan for you.