White blood cells (WBCs), or leukocytes, serve as primary defense mechanism of human body. They account for only 1% of total blood volume, These cells constantly patrol bloodstream and lymphatic system to identify, target, and destroy pathogens such as bacteria, viruses, and fungi.
Five Main Types of White Blood Cells
Leukocytes are categorized into two main groups based on their physical appearance under a microscope: Granulocytes (which contain small protein granules) and Agranulocytes (which do not).
1. Neutrophils
Neutrophils are most abundant type of WBC, making up majority of total count. They are the first to arrive at the site of an infection.
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Function: They kill bacteria and fungi through a process called phagocytosis (engulfing microorganism) and by releasing enzymes that break down the pathogen.
2. Lymphocytes
Lymphocytes act as adaptive immune system. They remembere previous microorganism so body can fight them off more effectively in future.
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B Cells: Produce antibodies to neutralize toxins and bacteria.
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T Cells: Directly attack infected host cells and coordinate the overall immune response.
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Natural Killer (NK) Cells: Target tumor cells and cells infected by viruses.
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3. Monocytes
Monocytes are largest size of white blood cells. They migrate from blood into tissues, they transform into macrophages.
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Function: It have a long lifespan than neutrophils and are responsible for cleaning up dead cells and debris, as well as attacking more persistent bacteria.
4. Eosinophils
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Function: It primarily target multicellular parasites and worms. They play a significant role in the triggering allergic reactions and managing inflammation.
5. Basophils
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Function: It secrete chemicals like histamine and heparin. Histamine dilates blood vessels to allow more immune cells to reach an infection site, while heparin prevents blood from clotting too quickly.
Normal Range and Clinical Significance
A standard Complete Blood Count (CBC) measures number of white blood cells in the blood. Results are typically reported as total count per microliter (mu L) of blood.
Normal Reference Ranges
| Cell Type | Percentage of Total WBC | Normal Range (Cells/μL) |
| Total WBC Count | 100% | 4,500 – 11,000 |
| Neutrophils | 40% – 60% | 2,500 – 7,000 |
| Lymphocytes | 20% – 40% | 1,000 – 4,800 |
| Monocytes | 2% – 8% | 200 – 600 |
| Eosinophils | 1% – 4% | 20 – 500 |
| Basophils | 0.5% – 1% | 10 – 100 |
Note: Normal ranges can also vary slightly depending on laboratory and individual’s age, sex, and environment.