White Blood Cells

White Blood Cells (WBCs), also known as leucocytes, are an essential component of the blood and immune system responsible for defending the body against infections, foreign invaders, and diseases. Unlike red blood cells (RBCs), which transport oxygen, white blood cells are primarily involved in protection and immunity. They circulate throughout the bloodstream and lymphatic system, identifying and neutralising pathogens such as bacteria, viruses, fungi, and parasites.
Though they constitute only about 1% of total blood volume, white blood cells play a vital role in maintaining health and immune balance.

Origin and Structure

All white blood cells originate from haematopoietic stem cells in the bone marrow, a process known as haematopoiesis. From there, they differentiate into various types of WBCs with specialised functions.
WBCs are nucleated cells (unlike red blood cells) and are larger in size, typically ranging from 7 to 20 micrometres. They can move independently by a process called amoeboid movement, allowing them to travel through capillary walls into tissues — a process known as diapedesis.
The average WBC count in healthy adults ranges between 4,000 and 11,000 cells per microlitre of blood. An increase in count is called leucocytosis, while a decrease is referred to as leucopenia.

Classification of White Blood Cells

White blood cells are classified into two major groups based on the presence or absence of granules in their cytoplasm:

1. Granulocytes

These cells contain visible granules in their cytoplasm and have multi-lobed nuclei. They are short-lived and form the first line of defence in immune response. Granulocytes include:
a) Neutrophils:

• Most abundant WBCs (50–70% of total).
• Nucleus has 2–5 lobes; granules stain faintly.
• Function: Act as phagocytes, engulfing and destroying bacteria and dead tissue through enzymes.
• Known as microphages, they form pus at sites of infection.

b) Eosinophils:

• Comprise 1–4% of total WBCs.
• Nucleus is bilobed; granules stain bright red with eosin dye.
• Function: Combat parasitic infections and regulate allergic responses by neutralising histamine.
• Increase in number during allergies, asthma, and parasitic infestations.

c) Basophils:

• Least abundant (<1% of total WBCs).
• Contain large, dark-staining granules rich in histamine and heparin.
• Function: Involved in inflammatory and allergic reactions.
  • Histamine dilates blood vessels and increases capillary permeability.
  • Heparin acts as an anticoagulant, preventing blood clotting within vessels.

2. Agranulocytes

These cells lack visible cytoplasmic granules and have single, large nuclei. They are generally longer-lived and perform specific immune functions.
a) Lymphocytes:

• Make up 20–40% of WBCs.
• Small, spherical cells with a large nucleus and little cytoplasm.
• Found in blood, lymphoid organs, and tissues.
• Types and functions:
  • B Lymphocytes (B cells): Produce antibodies (immunoglobulins) that neutralise pathogens.
  • T Lymphocytes (T cells): Provide cell-mediated immunity by directly destroying infected or cancerous cells.
  • Natural Killer (NK) Cells: Attack virus-infected cells and tumour cells without prior sensitisation.

b) Monocytes:

• Largest type of WBC (12–20 µm in diameter), accounting for 2–8% of the total count.
• Have a kidney-shaped nucleus.
• Function: Act as phagocytic cells, engulfing microorganisms and debris.
• When they migrate into tissues, they differentiate into macrophages, which play a key role in chronic inflammation and antigen presentation.

Formation and Lifespan

White blood cells are produced in the red bone marrow, lymph nodes, spleen, and thymus gland.

• Granulocytes: Formed in the bone marrow; live for a few hours to a few days.
• Lymphocytes: Produced in the bone marrow and matured in the thymus (T cells) or bone marrow (B cells); can live from weeks to years.
• Monocytes: Circulate in the blood for 1–2 days before migrating into tissues to become macrophages.

Functions of White Blood Cells

White blood cells collectively perform diverse yet complementary functions that ensure protection and repair:

1. Phagocytosis:
   • Neutrophils and monocytes engulf and digest foreign particles, pathogens, and dead cells.
2. Antibody Production:
   • B lymphocytes produce specific antibodies that target antigens (foreign molecules).
3. Cell-Mediated Immunity:
   • T lymphocytes identify and destroy infected or abnormal cells, such as cancer cells.
4. Inflammation Response:
   • Basophils and eosinophils release chemicals that mediate inflammation and allergic reactions.
5. Detoxification and Healing:
   • WBCs release enzymes and factors that neutralise toxins and promote tissue repair.
6. Immune Memory:
   • Some lymphocytes remain as memory cells, enabling faster immune response upon re-exposure to the same pathogen.

Disorders Related to White Blood Cells

1. Leucocytosis:
   • An abnormal increase in WBC count due to infections, inflammation, or stress.
2. Leucopenia:
   • A decrease in WBC count, often caused by viral infections, bone marrow suppression, or autoimmune diseases.
3. Leukaemia:
   • A type of blood cancer characterised by uncontrolled proliferation of immature WBCs in the bone marrow.
4. Agranulocytosis:
   • A severe reduction in granulocytes, leading to increased susceptibility to infection.
5. Lymphoma:
   • Cancer affecting lymphocytes and lymphoid tissues.

Diagnostic Importance

White blood cell count and differentiation form an integral part of routine blood tests (Complete Blood Count or CBC). The Differential Leucocyte Count (DLC) indicates the relative percentage of each WBC type, aiding in the diagnosis of infections and immune disorders:

• High neutrophil count: Indicates bacterial infection.
• High lymphocyte count: Suggests viral infection.
• High eosinophil count: Indicates allergies or parasitic infection.
• Low WBC count: Suggests bone marrow suppression or immune deficiency.

Summary of Major White Blood Cells

Importance in Human Health

White blood cells are the cornerstone of the immune defence system, maintaining the body’s resistance against pathogens and facilitating healing. Their ability to identify self and non-self cells safeguards against infections, autoimmune diseases, and malignancies.