Granulocytosis · What Is It, Causes, Treatments, and More

Published: Oct 17, 2025
Author: Alyssa Haag, MD•
Editor: Ahaana Singh•
Editor: Lisa Miklush, PhD, RN, CNS•
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What is granulocytosis?

Granulocytosis refers to an increased number of granulocytes circulating in the blood. Granulocytes are a type of white blood cell characterized by the presence of granules, or small particles, that enable their function. They can be further classified as mast cells, neutrophils, eosinophils, and basophils. Granulocytosis usually refers to an increase in neutrophils, eosinophils, or basophils.  

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What is the function of granulocytes?

Granulocytes assist the immune system in fighting off foreign invaders, such as viruses, bacteria, parasites, and fungi, that cause infection or inflammation. Neutrophils play a major role in fighting off bacterial infections; eosinophils play a major role in fighting off parasites; and basophils and mast cells play a major role in allergic reactions. 

Granulocytes develop in the bone marrow, the spongy-like substance inside the bone, and after entering the blood, they respond to signals sent from the immune system. When they encounter foreign substances invading the body, granulocytes undergo degranulation (i.e., the secretion of their specific granules in an effort to fight off the invaders). 

What causes granulocytosis?

Granulocytosis may be caused by bone marrow disorders and may also be associated with infections and autoimmune disorders.  

Granulocytosis is most commonly caused by bone marrow disorders where the production of granulocytes is increased. One of the most common bone marrow disorders resulting in granulocytosis is chronic myeloid leukemia, a type of white blood cell cancer. Other bone marrow-related disorders that can result in granulocytosis include: polycythemia vera, which primarily causes an increase in red blood cells; primary myelofibrosis, which causes destruction and consequent scarring of the bone marrow; and essential thrombocythemia, which results in an increase in platelet count.  

Infections, especially those caused by bacteria, can also cause granulocytosis. As part of the natural immune response, more neutrophils are produced in the bone marrow in order to fight off the bacteria, which may result in granulocytosis.  

Finally, autoimmune disorders, such as rheumatoid arthritis, may also cause granulocytosis. The chronic joint inflammation typical of rheumatoid arthritis often leads to elevated levels of circulating granulocytes.  

What are the signs and symptoms of granulocytosis?

The signs and symptoms of granulocytosis depend on the underlying cause. If it is the result of chronic myeloid leukemia, common signs and symptoms include abnormal bleeding, fatigue, loss of appetite, pale skin, night sweats, and frequent infections. In cases of rheumatoid arthritis-related granulocytosis, common signs and symptoms include joint pain, swelling, and stiffness.  

When granulocytosis is related to a bacterial infection, common signs and symptoms include fever, fatigue, headache, nausea, and vomiting. 

How is granulocytosis diagnosed?

Most often, granulocytosis is diagnosed by a blood test called a complete blood count (CBC). A CBC measures the amount of red blood cells, white blood cells, and platelets in an individual’s blood. Elevated white blood cell counts can indicate granulocytosis as well as an underlying disorder, disease, or infection. Similarly, elevated red blood cells or platelets can be indicative of polycythemia vera or essential thrombocythemia, respectively.  

How is granulocytosis treated?

Treatment for granulocytosis is usually targeted at treating the underlying cause. Managing the existing condition should help reduce the number of white blood cells in the blood and resolve the granulocytosis.  

What are the most important facts to know about granulocytosis?

Granulocytosis occurs when there is an increase in the number of granulocytes (e.g., neutrophils, basophils, or eosinophils) in the blood. Granulocytosis is usually a symptom of an underlying condition or disease, most commonly bacterial infections, autoimmune disorders, or bone marrow disorders. Diagnosis typically involves a complete blood count, and treatment is targeted at addressing the underlying cause. 

Key Takeaways

Definition 

Granulocytosis refers to an increased number of granulocytes (usually neutrophils, eosinophils, or basophils) circulating in the blood.  

Function of Granulocytes 

- Assist the immune system in fighting off foreign invaders  

     - Neutrophils: bacterial infections  

     - Eosinophils: parasites  

     - Basophils and mast cells: allergic reactions  

- Develop in the bone marrow → enter the blood → respond to signals from the immune system → encounter with foreign substances → degranulation  

Causes 

- Bone marrow disorders  

     - Chronic myeloid leukemia 

     - Polycythemia vera  

     - Primary myelofibrosis  

     - Essential thrombocythemia  

- Infections  

     - Especially bacterial infections  

- Autoimmune disorders  

     - E.g., rheumatoid arthritis  

Signs and Symptoms 

- Depend on the underlying cause 

     - Chronic myeloid leukemia: abnormal bleeding, fatigue, loss of appetite, pale skin, night sweats, frequent infections  

     - Rheumatoid arthritis: joint pain, swelling, stiffness  

     - Bacterial infection: fever, fatigue, headache, nausea, vomiting 

Diagnosis 

- Complete blood count  

     - A blood test measuring the amount of red blood cells, white blood cells, and platelets  

Treatment 

- Treat underlying cause 

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References

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Mitchell RN. Diseases of white blood cells, lymph nodes, spleen, and thymus. In: Kumar V, Abbas AK, Aster JC, eds. Robbins & Cotran Pathologic Basis of Disease. 10th ed. Elsevier; 2021:chap 13. 


Wanahita A, Goldsmith EA, Musher DM. Conditions associated with leukocytosis in a tertiary care hospital, with particular attention to the role of infection caused by clostridium difficile. Clin Infect Dis. 2002;34(12):1585-1592. doi:10.1086/340536