Segmented Granulocyte – Definition & Function
Segmented granulocytes are mature white blood cells with a multi-lobed nucleus. They play a key role in defending the body against bacterial and fungal infections.
Things worth knowing about "Segmented Granulocyte"
Segmented granulocytes are mature white blood cells with a multi-lobed nucleus. They play a key role in defending the body against bacterial and fungal infections.
What Are Segmented Granulocytes?
Segmented granulocytes (also called segmented neutrophils or polymorphonuclear neutrophils) are the most mature and most abundant type of neutrophilic granulocyte found in human blood. Their defining feature is a multi-lobed (segmented) nucleus, typically consisting of two to five lobes connected by thin chromatin strands. They belong to the group of white blood cells (leukocytes) and form a critical component of the innate immune system.
In a differential blood count, segmented granulocytes account for approximately 50 to 70 percent of all leukocytes in a healthy adult. Their lifespan in the bloodstream is only a few hours to days, which is why the bone marrow must continuously produce new cells.
Formation and Maturation
Segmented granulocytes are produced in the bone marrow through a process called granulopoiesis. Precursor cells pass through several maturation stages:
- Myeloblast – earliest recognizable precursor
- Promyelocyte – first granules are formed
- Myelocyte – further differentiation
- Metamyelocyte – nuclear shape begins to change
- Band (stab) neutrophil – immature form with a band-shaped nucleus
- Segmented granulocyte – mature form with a segmented nucleus
The transition from the band form to the segmented form is a key indicator of cellular maturity. During high demand, such as during a bacterial infection, immature band forms may be released into the bloodstream in greater numbers -- a finding known as a left shift.
Function and Mechanism of Action
Segmented granulocytes represent the first line of cellular defense against invading pathogens, particularly bacteria and fungi. Their key mechanisms include:
- Phagocytosis: They engulf and digest pathogens and cellular debris.
- Degranulation: They release antimicrobial substances stored in their granules, including enzymes such as myeloperoxidase and elastase.
- Neutrophil Extracellular Traps (NETs): They expel web-like structures of DNA and proteins that trap and kill pathogens.
- Chemotaxis: They migrate toward sites of inflammation in response to chemical signals (cytokines and chemokines).
Clinical Significance – Changes in the Blood Count
Elevated Levels (Neutrophilia)
An increase in segmented granulocytes is called neutrophilia and may indicate:
- Bacterial infections (e.g., pneumonia, appendicitis)
- Inflammatory conditions (e.g., rheumatoid arthritis)
- Tissue damage (e.g., after myocardial infarction or surgery)
- Certain medications (e.g., corticosteroids)
- Myeloproliferative disorders (e.g., chronic myeloid leukemia)
Decreased Levels (Neutropenia)
A reduction in granulocyte count is referred to as neutropenia. In severe cases, this is termed agranulocytosis, which is associated with a significantly increased risk of infection. Possible causes include:
- Viral infections (e.g., influenza, HIV)
- Bone marrow disease or damage
- Chemotherapy or radiation therapy
- Autoimmune conditions
- Certain medications (e.g., metamizole, thiamazole)
Diagnosis
Segmented granulocytes are measured as part of a complete blood count (CBC) with differential, which determines the percentage and absolute count of each leukocyte type. Modern laboratory analyzers perform this automatically; abnormal results are typically followed by manual microscopic review by a qualified physician.
Reference values for adults: segmented neutrophils 1,800 to 7,500 cells per microliter of blood (absolute) or 50 to 70 percent of total leukocytes.
References
- Hoffbrand, A.V. & Moss, P.A.H. – Hoffbrand's Essential Haematology, 7th edition. Wiley-Blackwell, Oxford, 2016.
- Silbernagl, S. & Lang, F. – Color Atlas of Pathophysiology. Georg Thieme Verlag, Stuttgart, 2020.
- World Health Organization (WHO) – Blood Safety and Clinical Technology: Guidelines on Standard Operating Procedures for Haematology. WHO, Geneva, 2009.
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