Chapter 17B: Blood

Leukocytes (White Blood Cells)

Overview of Leukocytes

Leukocytes, also known as white blood cells (WBCs), are a critical component of the blood, responsible for defending the body against infectious disease and foreign invaders. Unlike other formed elements in blood, leukocytes are complete cells, possessing nuclei and organelles.

• Definition: Leukocytes are the only formed elements in blood that are complete cells with nuclei and organelles.

• Proportion: They make up less than 1% of total blood volume.

• Normal Count: 4,800 to 10,800 WBCs per microliter (μl) of blood.

• Function: Defense against disease; an increase in WBC count is a normal response to infection.

Classification of Leukocytes

Leukocytes are classified into two main categories based on the presence or absence of visible cytoplasmic granules:

• Granulocytes: Contain visible cytoplasmic granules. Types include neutrophils, eosinophils, and basophils.

• Agranulocytes: Lack visible cytoplasmic granules. Types include lymphocytes and monocytes.

Mnemonic for abundance: Never Let Monkeys Eat Bananas (Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils).

Types and Relative Percentages of Leukocytes in Normal Blood

The following table summarizes the types and relative percentages of leukocytes found in normal blood:

Leukocyte Type	Category	Relative Percentage (%)
Neutrophils	Granulocyte	50–70
Eosinophils	Granulocyte	2–4
Basophils	Granulocyte	0.5–1
Lymphocytes	Agranulocyte	25–45
Monocytes	Agranulocyte	3–8

Leukocyte Morphology

Leukocytes vary in appearance, which aids in their identification under the microscope:

• Neutrophils: Multilobed nucleus, pale red and blue cytoplasmic granules.

• Eosinophils: Bilobed nucleus, red cytoplasmic granules.

• Basophils: Bilobed nucleus, purplish-black cytoplasmic granules.

• Lymphocytes: Large spherical nucleus, thin rim of pale blue cytoplasm.

• Monocytes: Kidney-shaped nucleus, abundant pale blue cytoplasm.

Example: A blood smear stained and viewed under a microscope will show these distinct features, allowing for differential WBC counts and diagnosis of various conditions.

Additional info: The images provided in the slides visually demonstrate the morphological differences between the types of leukocytes, which is essential for laboratory identification and clinical diagnostics.