White Blood Cells - Part 1: Granulocytes & Monocytes

Learning Outcomes

• Knowledge of the functions of white blood cells, including their roles in immunity, phagocytosis, and inflammatory responses.

• Knowledge of the microscopic appearance of the white blood cells, enabling identification and differentiation of each type.

• Understanding of the representation of white blood cells in the full blood count, including absolute and relative counts.

• Understanding of the indications for disease diagnosis that the differential white blood cell count provides, such as infections, allergies, and immune disorders.

Overview

• Each type of white blood cell (WBC) has a specific function, contributing to the body's defense mechanisms.

• Differential diagnosis can often be suggested based on:

  • How WBCs are represented in the full blood count (FBC), providing quantitative data.

  • Their microscopic appearance, offering qualitative insights.

Basic Function

• WBCs are responsible for immunity, mainly through phagocytosis, which is crucial for clearing pathogens and debris.

• Phagocytosis: Ingestion and destruction of foreign and unwanted material like bacteria, old red blood cells, and cellular debris.

• Phagocytes: Cells that perform phagocytosis, primarily neutrophils, monocytes, and macrophages, each with specific roles and mechanisms.

Leukocytes

• WBCs, or leukocytes, are cellular components of the immune system, defending the body against infections and foreign substances.

• The body maintains a specific concentration of WBCs in peripheral blood for efficient immune function, ensuring a rapid response to threats.

• Adult reference range: WBC 4 – 11 \times 10^9/L.

• Hematology analyzer provides automated white cell count with parameters like:

  • WBC High: Leucocytosis, indicating infection, inflammation, or other conditions.

  • WBC Low: Leucopenia, suggesting immune suppression or bone marrow disorders.

Haematopoiesis

• With the exception of T cells, all WBCs are produced and mature in the Bone Marrow (BM), the primary site of hematopoiesis.

• The BM contains a reservoir of immature WBCs to supplement the peripheral blood population when needed, ensuring a continuous supply of immune cells.

• The BM contains more WBCs than RBCs, reflecting the high turnover and demand for immune cells.

• Myeloid lineage development:

  • Multipotent stem cell → Common myeloid progenitor, the precursor for granulocytes, monocytes, and macrophages.

Granulocytes

• Contain cytoplasmic granules, which are filled with enzymes and antimicrobial substances.

• Effector cells: Mature, fully functional cells capable of performing specific immune functions.

• In healthy individuals, only mature effector cells are present in the peripheral blood; reserves of developing cells remain in the BM, ready for rapid deployment.

Neutrophils

• Approximately 10^{11} neutrophils are produced daily by the BM, highlighting their critical role in immune defense.

• Short lifespan: 6 hours - 5 days circulating in the peripheral blood (PB) and marginating in reticuloendothelial tissues, allowing for rapid response to infections.

• Primary cellular component of the innate immune system, providing immediate and non-specific defense against pathogens.

Neutrophil Response

• Highly sensitive to chemotactic stimuli, enabling rapid migration to sites of infection and inflammation.

• Rapid migration to the site of inflammation, directed by chemical signals released by infected tissues.

• Marginating cells are available for immediate mobilization, adhering to blood vessel walls and ready to enter tissues.

• 'First responders' of the immune system, quickly arriving at the site of infection to phagocytose pathogens and release antimicrobial substances.

Neutrophil Count

• Most abundant WBC species, typically comprising 50-70% of circulating WBCs.

• Neutrophilia: High neutrophil count, often indicative of bacterial infection or inflammation.

• Neutropenia: Low neutrophil count, increasing the risk of infection.

• Reference ranges:

  • WBC: 4.0 – 11.0 \times 10^9/L

  • NEUT (Absolute count): 2.0 – 8.0 \times 10^9/L

  • NEUT (Percentage): 50 – 70%

Neutrophil Morphology

• 3 - 5 nuclear lobes joined by chromatin bridges, a distinctive feature for identification.

• Azurophilic cytoplasmic granules, containing enzymes and antimicrobial substances.

Neutrophil Granules

• Primary granules:

  • Myeloperoxidase (MPO), involved in the production of hypochlorous acid, a potent antimicrobial agent.

  • Bacterial permeability inducing factor (BPI) – effective against gram-negative bacteria, disrupting their cell membranes.

  • Lysozyme – effective against gram-positive bacteria, breaking down their cell walls.

  • Elastase – effective against gram-positive, gram-negative, fungi, enterotoxins, degrading proteins and contributing to tissue damage.

  • Acid hydrolases, involved in the breakdown of phagocytosed material.

• Secondary granules:

  • Lactoferrin – iron-binding; bacteriostatic and bactericidal, depriving bacteria of essential iron.

• Tertiary granules:

  • Gelatinase (MMP) – digests vessel basement membrane, facilitating migration through tissues.

• Granule release pattern during granulopoiesis (granule development), ensuring appropriate enzyme availability.

Eosinophils

• Principally involved in immunity against helminth infections (roundworm, hookworm, Schistosomiasis), releasing toxic substances to kill parasites.

• Contribute to hypersensitivity reactions, such as asthma and allergies, by releasing inflammatory mediators.

• Develop in the BM, and mature effector cells are released into the PB, ready to migrate to sites of inflammation.

• BM reserve, but no marginal reserve in periphery, indicating limited availability for immediate mobilization.

Eosinophil Count

• Minor granulocyte species, typically comprising 1-3% of circulating WBCs.

• Eosinophilia: High eosinophil count, often associated with parasitic infections or allergic reactions.

• Reference range:

  • EOSO (Absolute count): 0.0 – 0.5 \times 10^9/L

  • WBC: 4.0 – 11.0 \times 10^9/L

Eosinophil Morphology

• Bilobed nucleus, a characteristic feature for identification.

• Large orange-red cytoplasmic granules, containing toxic substances.

Eosinophil Granules

• Major basic protein – disrupts the lipid bilayer of parasites, causing cell lysis.

• Eosinophil peroxidase – bactericidal, producing reactive oxygen species.

• Eosinophil cationic protein – helminthotoxic and bactericidal, damaging parasite cell membranes.

• Eosinophil-derived neurotoxin – helminthotoxic and bactericidal, affecting parasite nerve function.

Basophils

• Principally involved in hypersensitivity reactions (allergy), releasing histamine and other inflammatory mediators.

• Contribute to anti-helminth immunity, although their role is less prominent than eosinophils.

• Develop in the BM, and mature effector cells are released into the PB, where they circulate and respond to inflammatory signals.

• BM reserve, but no marginal reserve in periphery, suggesting limited immediate availability.

Basophil Count

• Least abundant granulocyte species, typically comprising less than 1% of circulating WBCs.

• Basophilia: High basophil count, often associated with allergic reactions or myeloproliferative disorders.

• Reference range:

  • BASO (Absolute count): 0.0 – 0.1 \times 10^9/L

  • WBC: 4.0 – 11.0 \times 10^9/L

Basophil Morphology

• Indented or bilobed nucleus, often obscured by granules.

• Abundant large purple-black cytoplasmic granules, containing various inflammatory mediators.

• Contain mucopolysaccharides (e.g., Heparin) and Histamine (involved in inflammation and vascular permeability), contributing to allergic reactions.

Monocytes

• Not terminally differentiated 'effector' cells, but rather precursors to macrophages and dendritic cells.

• Bloodborne stage in the development of tissue macrophages, circulating in the bloodstream before migrating into tissues.

• Circulate in PB for 2 – 3 days, during which they can respond to inflammatory signals.

• Migrate out of blood vessels into surrounding tissues and differentiate into macrophages, which are phagocytic cells that remove debris and pathogens.

• Macrophage → 'effector' cell, also known as histiocytes, residing in tissues and performing immune functions.

• Macrophage/histiocyte lifespan can be several months, allowing for long-term immune surveillance.

Monocyte Count

• Myeloid cell but NOT a granulocyte, distinguishing them from neutrophils, eosinophils, and basophils.

• Monocytosis: High monocyte count, often associated with chronic infections, inflammation, or malignancy.

• Monocytopenia: Low monocyte count (rare), suggesting immune suppression or bone marrow failure.

• Reference ranges:

  • MONO (Absolute count): 0.4 – 1.0 \times 10^9/L

  • WBC: 4.0 – 11.0 \times 10^9/L

Monocyte Morphology

• Highly indented 'C shaped' nucleus, a characteristic feature for identification.

• Pale greyish-blue cytoplasm, often containing vacuoles.

• Few small azurophilic cytoplasmic granules, containing enzymes and antimicrobial substances.

• Contain digestive enzymes relevant for macrophage function, enabling the breakdown of phagocytosed material.

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