ACTIVE RECALL

• Neutrophils


  A type of white blood cell critical for the immune response
• Neutrophils


  • Present in the peripheral blood in two forms: segmented and band neutrophils
  • Segmented neutrophils constitute the majority of circulating leukocytes
• Neutrophil Development

  1. Location: Bone marrow

  2. Common Progenitor: Granulocyte-monocyte progenitor (GMP)

  3. Granulocyte colony-stimulating factor (G-CSF) -> stimulating cytokine
• Developmental Pools (in the BM where developing neutrophils are found)

  • Stem Cell Pool = hematopoietic stem cells (HSCs) capable of self-renewal and differentiation
  • Proliferation (Mitotic) Pool: actively dividing cells (Common myeloid progenitors (CMPs), Granulocyte-macrophage progenitors (GMPs), Myeloblasts, Promyelocytes, Myelocytes)
  • Maturation (Storage) Pool: Cells undergo nuclear maturation and form the marrow reserve, available for release (Metamyelocytes, Band neutrophils, Segmented neutrophils)
• Stages of Neutrophil Development

  1. Myeloblasts

  2. Promyelocytes

  3. Myelocytes

  4. Metamyelocytes

  5. Band Neutrophils

  6. Segmented Neutrophils
• Myeloblasts


  • Comprises 0-3% of nucleated cells in BM
  • Size: 14-20 µm
  • Types: Type I (High nucleus-to-cytoplasm ratio, slightly basophilic cytoplasm, fine nuclear chromatin, 2-4 nucleoli, no visible granules), Type II (Dispersed primary granules present, up to 20 granules), Type III (More than 20 granules, darker chromatin and more purple cytoplasm)
• Promyelocytes

  • 1-5% of this composes the nucleated cells in BM
  • Size: 16-25 µm (relatively larger)
  • Round to oval eccentric nucleus
  • 1-3 nucleoli that may obscure the nucleus
  • Chromatin clumping or heterochromatin around the edges of nucleus
  • Presence of paranuclear halo or hof
  • Evenly basophilic cytoplasm with full azurophilic granules
• Myelocytes


  • Makes up the 6-17% of nucleated cells in BM
  • Final stage of mitosis/cell division
  • Ceasing of azurophilic granule production= onset of secondary (specific) granule production
  • Cytoplasm is more lavender pink than blue
  • Composed of: b2-Microglobulin, Collagenase, Gelatinase, Lactoferrin, Neutrophil gelatinase-associated lipocalin, Transcobalamin
• Stages of Myelocytes

  1. Early Myelocytes: Similar morphology to promyelocytes but with visible secondary granules (grainy pale pink patches of the cytoplasm concentrated at the Golgi Apparatus) -> dawn of neutrophilia

  2. Late Myelocytes: Smaller, more heterochromatin, difficult-to-see nucleoli
• Metamyelocytes

  • Comprises 3-20% of nucleated cells in BM
  • No longer capable of division
  • Nucleus indented (kidney/peanut-shaped)
  • Increasing chromatin clumping
  • No nucleoli present
  • Synthesis of tertiary (gelatinase) granule synthesis, little to no basophilic cytoplasm due to very little residual RNA
• Band Neutrophils

  • 9-32% in the BM
  • Peripheral Blood: 0-5%
  • No cytoplasmic RNA
  • Tertiary granules continue forming
  • Formation of secretory granules (vesicles) may begin
  • Highly clumped nucleus with indentation exceeding half the diameter
• Segmented Neutrophils

  • Percentage in Bone Marrow: 7-30%
  • Peripheral Blood: 50-70% of leukocytes in adults (highest number)
  • Differs from bands by the presence of between 2-5 nuclear lobes connected by thread-like filaments
  • Secretory granules continue forming, nucleus has 2-5 lobes connected by filaments, highest numbers in peripheral blood
• Neutrophil Production

  Rate of Production: 0.9 to 1.0 × 10^9 cells/kg per day<|>Proliferative Pool: Contains approximately 2.1 × 10^9 cells/kg<|>Maturation Pool: Contains approximately 5.6 × 10^9 cells/kg, representing a 5-day supply
• Neutrophil Development Timeline

  1. From (HSC) to Myeloblast: Transit time not precisely measured

  2. From Myeloblast to Myelocyte: approximately 6 days

  3. Maturation Pool (Myelocyte to Segmented Neutrophil): Transit time is approximately 4 to 6 days

  4. Granulocyte release from the bone marrow is stimulated by granulocyte colony-stimulating factor (G-CSF)
• Peripheral Blood Pools

  • Circulating Neutrophil Pool (CNP): Neutrophils freely circulate in the bloodstream
  • Marginated Neutrophil Pool (MNP): Neutrophils are loosely attached to the walls of capillaries in tissues such as the liver, spleen, and lung
• Neutrophil Kinetics

  1. Entry to Peripheral Blood: Neutrophils have a short half-life of approximately 7 hours in the peripheral blood

  2. Diapedesis: Migration of neutrophils from PB to tissue (Stimulated by integrins and selectins, a process by which neutrophils migrate through the endothelial layer into tissues)

  3. Entry of Neutrophils in the Tissue: Non-infectious/inflammatory conditions: Lifespan measured in hours, Infection and Inflammation: Variable and prolonged by anti-apoptotic signals
• Overview of Innate Immunity

  • Destruction of foreign organisms in a non-antigen-specific manner
  • No protection against re-exposure to the same pathogen
  • Relies on barriers provided by skin and mucous membranes
  • Involves phagocytes such as neutrophils and monocytes
  • Includes the complement system, a humoral component
• Major Function of Neutrophils

  Phagocytosis and Destruction of Foreign Materials and Microorganisms
• Key Processes of Neutrophils

  1. Seeking: Chemotaxis (Neutrophils are attracted to the site of infection by chemotactic agents), Motility (Neutrophils move toward the site of infection), Diapedesis (Neutrophils exit the bloodstream and enter the tissues)

  2. Destruction: Phagocytosis (Neutrophils engulf and digest foreign material and microorganisms)
• Neutrophil Recruitment to Inflammatory Sites

  Binding of chemotactic agents -> neutrophil receptors (Produced by microorganisms, damaged cells, or other leukocytes)
• Neutrophil Response

  1. Rolling (Neutrophils roll along endothelial cells using selectins)

  2. Adhesion (Neutrophils use integrins to bind tightly to endothelial cells)

  3. Diapedesis (Neutrophils transmigrate through endothelial cells using integrins and associated proteins)

  4. Release of Tertiary Granules (Contains gelatinase and collagenase to degrade collagen and activate chemokines like IL-8)

  5. Phagocytosis Process (at the site of inflammation/infection)
• Phagocytosis Process

  1. Recognition and Attachment (Neutrophils recognize pathogens directly or via opsonic molecules)

  2. Engulfment (Neutrophils form pseudopodia around the pathogen, creating a phagosome)

  3. Destruction (Formation of the phagosome allows the reduced nicotinamide adenine dinucleotide (NADH) oxidase complex within the phagosome membrane to assemble -> generation of reactive oxygen species, Myeloperoxidase converts hydrogen peroxide to hypochlorite, Granule Fusion: Primary and secondary granules fuse with the phagosome, releasing bactericidal molecules)

  4. Extracellular Release of Granule Contents (Granules release contents into the extracellular matrix, functions as chemotactic agents and stimulates macrophages to phagocytize dead neutrophils, may cause tissue damage as inflammatory agents)
• Second Function of Neutrophils

  Generation of Neutrophil Extracellular Traps (NETs): Threadlike structures composed of nucleosomes from unfolded nuclear chromatin material (DNA) with attached enzymes, Trap and kill gram-positive and gram-negative bacteria, as well as fungi, NETosis: A form of cell death resulting in the release of NETs
• Third Function of Neutrophils

  Secretory Function: Secretes or a source of Transcobalamin I (R binder protein) necessary for proper absorption of vitamin B12, Cytokines: Neutrophils are a source of various cytokines