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Granulocytes – Function, Types and Reference Values

Granulocytes are white blood cells that play a central role in the immune system. They fight infections and are involved in inflammatory responses.

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Things worth knowing about "Granulocytes"

Granulocytes are white blood cells that play a central role in the immune system. They fight infections and are involved in inflammatory responses.

What Are Granulocytes?

Granulocytes are a group of white blood cells (leukocytes) and among the most important cells of the innate immune system. Their name derives from the characteristic granules found in their cytoplasm, which contain various substances used during immune defense. Granulocytes are produced in the bone marrow, circulate in the bloodstream, and migrate into tissues when needed.

Types of Granulocytes

Based on their staining properties in the laboratory, three main types are distinguished:

Neutrophilic granulocytes (Neutrophils): Accounting for approximately 50–70% of all leukocytes, they are the primary defense line against bacterial and fungal infections. They eliminate pathogens through phagocytosis (ingestion and digestion) and by releasing antimicrobial substances.
Eosinophilic granulocytes (Eosinophils): Making up about 2–4% of leukocytes, they play an important role in defense against parasitic infections and in allergic reactions and asthma. Elevated levels can indicate an allergy or parasitic disease.
Basophilic granulocytes (Basophils): With a proportion of less than 1%, they are the rarest granulocytes. They release histamine and other inflammatory mediators and are significantly involved in immediate-type allergic reactions (type I hypersensitivity).

Function in the Immune System

Granulocytes are essential for the body's rapid response to invading pathogens. Once an infection or injury is detected, they are recruited to the site by chemical signaling molecules called cytokines and chemokines. At the site, they exert their effects through:

Phagocytosis: Ingestion and destruction of bacteria, fungi, and other foreign materials.
Degranulation: Release of enzymes and proteins from granules to combat pathogens and regulate inflammation.
Formation of Neutrophil Extracellular Traps (NETs): Neutrophils can expel web-like structures made of DNA and proteins that trap and kill bacteria.

Reference Values and Laboratory Diagnostics

The number of granulocytes is determined as part of a complete blood count (CBC) with differential. Reference values for neutrophilic granulocytes in adults typically range between 1,800 and 7,500 cells per microliter of blood.

Granulocytosis (elevated levels): May indicate bacterial infections, inflammation, physical stress, or certain medications. In rare cases, an extreme increase may suggest leukemia.
Granulocytopenia / Neutropenia (reduced levels): A deficiency in granulocytes significantly increases susceptibility to infections. Causes may include chemotherapy, certain medications, autoimmune diseases, or bone marrow disorders.

Clinical Significance

Disorders affecting the number or function of granulocytes have significant clinical consequences. Severe neutropenia (neutrophil count below 500 cells/µl) is considered a medical emergency, as even normally harmless microorganisms can trigger life-threatening infections. Patients undergoing chemotherapy are particularly at risk. In such situations, granulocyte colony-stimulating factors (G-CSF) may be used to stimulate the production of granulocytes in the bone marrow.

References

Abbas, A.K., Lichtman, A.H., Pillai, S. - Cellular and Molecular Immunology. Elsevier, 10th edition (2021).
Longo, D.L. et al. - Harrison's Principles of Internal Medicine. McGraw-Hill, 21st edition (2022).
Mayadas, T.N., Cullere, X., Lowell, C.A. - The multifaceted functions of neutrophils. Annual Review of Pathology: Mechanisms of Disease, 9:181-218 (2014).

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