White Blood Cells (WBCs) – Comprehensive Notes

White Blood Cells (WBCs) – Overview

  • Also called leucocytes; nucleated formed elements of blood involved in defense mechanisms.
  • They constitute a small fraction of blood cells: a little more than frac{1}{10} imes 1 ext{%} = 0.1 ext{%}; in practice 1–40% depending on lineage and context, but typically around 1% of blood cells when counting all leukocytes.
  • Histologically distinct types: major cell lineages include neutrophils, eosinophils, basophils (granulocytes), monocytes (precursors to macrophages), and lymphocytes (T, B, NK).
  • Key features across WBCs: presence of nucleus, organelles (mitochondria, RER, Golgi in some), and cytoplasmic granules (in granulocytes).
  • Primary functions: defense against infection, inflammation, antigen presentation, antibody production, cytotoxic responses, and regulation of immune responses.

Neutrophils (PMNs) – General Features

  • Most abundant circulating WBC: approximately 60%−70%60\%-70\% of circulating leukocytes.
  • Lifespan: short in blood ∼6−8 hours\sim 6-8\text{ hours}; in tissue ∼1−4 days\sim 1-4\text{ days}.
  • First responders in acute inflammation; form the 1st line of defense along with monocytes.
  • Nucleus: multi-lobed (2–5 lobes) with thin chromatin strands connecting lobes; more lobes indicate an older neutrophil.
  • Cytoplasm: light pink/neutral staining; contains two main granule types:
    • Primary (azurophilic) granules – large, reddish-purple
    • Secondary (specific) granules – smaller, paler
  • Morphology descriptor: often termed polymorphonuclear leukocytes (PMNs).
Neutrophil Functions
  • Phagocytosis of bacteria and cellular debris.
  • Respiratory burst producing reactive oxygen species.
  • Release of granule enzymes for microbial killing.
  • Formation of neutrophil extracellular traps (NETs): chromatin fibers plus enzymes that trap microbes.
  • Recognize bacteria via lipopolysaccharides (LPS) in bacterial membranes; recognize dead cells via exposed internal phospholipids.
  • Chemotaxis: attracted by histamine, complement fragments, bacterial LPS; leukotrienes released by early neutrophils recruit more neutrophils and eosinophils.
  • Basement membrane penetration: tertiary granules contain gelatinase aiding penetration of basement membranes.
  • Diapedesis and ameboid movement enable migration to infection sites.
Special Histology Clues & Clinical Relevance
  • Barr body (drumstick) in neutrophil nucleus indicates inactive X chromosome (seen in females).
  • Left shift: increased band forms indicating acute infection.
  • Toxic granulation and Döhle bodies indicate infection/inflammation.
  • Neutrophilia: bacterial infections, inflammation, stress.
  • Neutropenia: chemotherapy, aplastic anemia, overwhelming infection.
  • Chronic granulomatous disease: defective NADPH oxidase → impaired respiratory burst.
Neutrophil Granules & Contents

  • Primary (Azurophilic) Granules

    • Contain Myeloperoxidase (MPO)
    • Enzyme: MPO converts hydrogen peroxide and chloride to hypochlorous acid, a potent microbicidal agent
    • Reaction (MPO-catalyzed):
      H<em>2O</em>2+Cl−→MPOHOCl+H2O\mathrm{H<em>2O</em>2} + \mathrm{Cl^-} \xrightarrow{\text{MPO}} \mathrm{HOCl} + \mathrm{H_2O}
    • Defensins; acid hydrolases, lysozyme, elastase, cathepsin G

  • Secondary (Specific) Granules

    • Contain lactoferrin, lysozyme, collagenase, and components for the NADPH oxidase system (respiratory burst)

  • Tertiary Granules

    • Contain gelatinase (MMP-9) and cathepsins; facilitate migration through extracellular matrix

  • Key phrases: “granules” confer bactericidal activity inside phagosomes; oxidase components support intracellular kill and extracellular matrix remodeling for migration.

Eosinophils – General Features & Function

  • Type: Granulocyte; relative count 1%−4%1\%-4\% of circulating leukocytes.
  • Lifespan: about 5 hours5\text{ hours} in blood, then a few days in tissues.
  • Size: 10−20 μm10-20\ \mu m.
  • Nucleus: Bilobed (often resembles spectacles or eyeglasses).
  • Cytoplasm: Large, coarse eosinophilic (red-orange) granules.
  • Granules
    • Specific (secondary) granules: large, acidophilic; contain major basic protein (MBP), eosinophil cationic protein, eosinophil peroxidase, and eosinophil-derived neurotoxin; MBP is toxic to parasites (especially helminths).
    • Azurophilic (primary) granules: lysosomal enzymes (acid hydrolases).
Eosinophil Functions
  • Defense against parasitic infections (especially helminths).
  • Modulation of allergic reactions by counteracting histamine and leukotrienes released by basophils and mast cells.
  • Phagocytosis of antigen–antibody complexes.
  • Release enzymes (histaminase, arylsulfatase) to counteract basophil/mast cell mediators.
  • Histology: cytoplasm stains bright red-orange with H&E; eosinophilic granules prominent with Wright-Giemsa stain.
Eosinophil Mediators & Scenarios
  • Substances secreted: eosinophil peroxidase, MBP, eosinophil-derived neurotoxin, interleukins 4 and 5, phospholipases, ribonuclease, Cathepsins.
  • Mechanism against parasites: cytotoxic granules and enzymes attack nematodes and platyhelminths.

Basophils – General Features & Function

  • Rarest WBC: <1% of circulating leukocytes.
  • Size: 10−14 μm10-14\ \mu m (slightly larger than RBCs).
  • Nucleus: typically bilobed or S-shaped; nucleus may be obscured by dense granules.
  • Cytoplasm: basophilic granules (dark blue-purple) that can overlie the nucleus.
  • Granule contents: histamine (vasodilation, increased vascular permeability), heparin (anticoagulant), heparan sulfate, chondroitin sulfate, peroxidase, other mediators; leukotrienes (LTC4, LTD4, LTE4) – slow-reacting substance of anaphylaxis.
Basophil Functions & Receptors
  • Key player in Type I hypersensitivity (allergic) reactions.
  • FcεRI: high-affinity receptor for IgE; binding of allergen cross-links IgE → degranulation.
  • Work alongside tissue counterparts (mast cells).
  • Basophils secrete cytokines that recruit eosinophils.
  • Distinguishing features vs other granulocytes: neutrophils have multilobed nuclei with pale granules; eosinophils have bilobed nucleus with red/orange granules; basophils have obscured bilobed nucleus with dark purple granules.

Monocytes – General Features & Function

  • Largest circulating leukocyte: 12−20 μm12-20\ \mu m.
  • Represent about 5%5\% of WBCs; mature monocytes spend a few hours in blood before migrating into tissues.
  • Nucleus: large, kidney-shaped or horseshoe-shaped, often eccentric.
  • Cytoplasm: abundant, pale blue-gray with a ground-glass appearance.
  • Granules: fine azurophilic granules (lysosomes); less prominent than in neutrophils.
Monocyte Functions
  • Precursors of macrophages in tissues (e.g., macrophages, osteoclasts, Kupffer cells, microglia).
  • Phagocytosis: engulf pathogens, apoptotic cells, and debris.
  • Antigen presentation: express MHC class II and present antigens to CD4+ T cells.
  • Cytokine production: secrete IL-1, TNF-α, and other inflammatory mediators.
  • Action mechanism: monocytes leave blood to become macrophages; macrophages perform antigen presentation and tissue repair functions; can secrete growth factors and attract lymphocytes via lymphokines.
  • Bacterial endotoxins can induce tissue thromboplastin expression in monocytes, linking to clotting and potential toxic shock.

Lymphocytes – General Features & Subtypes

  • Type: Agranulocytes; typically 20%−40%20\%-40\% of WBCs.
  • Size: small to large; 6−9 μm6-9\ \mu m to 10−14 μm10-14\ \mu m.
  • Nucleus: round, dense, darkly staining; high nucleus-to-cytoplasm ratio.
  • Cytoplasm: thin rim of pale blue around nucleus; few or no granules (occasionally azurophilic granules).
  • Subtypes:
    • Small lymphocytes: mostly inactive T cells and B cells (older cells); round with scant cytoplasm.
    • Large lymphocytes: activated immunoblasts (younger cells); more abundant cytoplasm; includes natural killer (NK) cells.
Lymphocyte Identification & Function
  • Distinguishing B vs T cells by light microscopy is difficult; use immunohistochemistry:
    • CD3 → T cells
    • CD19/CD20 → B cells
    • CD16/CD56 → NK cells
  • Functions:
    • T lymphocytes: cell-mediated immunity (cytotoxic T cells, helper T cells, regulatory T cells) – about 75%75\% of lymphocytes.
    • B lymphocytes: humoral immunity; differentiate into plasma cells → antibody secretion.
    • NK cells: innate immunity; kill virus-infected and tumor cells.
  • T-helper cells (CD4+): activate B-lymphocytes.
  • Regulatory T cells suppress anti-self Th lymphocytes.
  • Cytotoxic T cells (CD8+): kill infected cells.
  • Lymphocytes persist through activation with B cells becoming plasma cells in germinal centers; possible germinal center reactions lead to antibody class switching and memory formation.

Plasma Cells – Origin, Morphology, & Function

  • Origin: derived from B lymphocytes after antigen stimulation.
  • Role: terminally differentiated antibody-secreting cells (plasma cells).
  • Morphology:
    • Oval or round cell; cytoplasm is basophilic due to abundant rough endoplasmic reticulum (RER).
    • Nucleus eccentrically placed with a clock-face (cartwheel) chromatin pattern.
    • Prominent Golgi apparatus with a perinuclear hof (clear zone next to nucleus).
  • Location: commonly in connective tissue, lamina propria of gut, lymphoid tissues (lymph nodes, spleen), and sites of chronic inflammation.
  • Function: factory for antibodies (immunoglobulins) tailored to the antigen that stimulated the B cell.
  • Immunoglobulin isotypes (antibody classes): IgM, IgG, IgA, IgE; class switching leads to predominant IgG, IgA, etc., depending on signals in the germinal center.
  • Lifespan: typically 10−30 days10-30\ days; some become long-lived plasma cells in bone marrow.

Natural Killer (NK) Cells – Overview & Function

  • Type: Large granular lymphocytes (LGLs) that are central to innate immunity.
  • Morphology:
    • Size: larger than small lymphocytes.
    • Cytoplasm: azurophilic granules (purple with H&E) containing perforin and granzymes.
    • Nucleus: round to slightly indented; chromatin less condensed than small lymphocytes.
    • Appearance: collectively termed large granular lymphocytes.
  • Markers & Identity:
    • CD56+ (NCAM) and CD16+ (FcγRIII)
    • Lack CD3 and TCR (CD3−, TCR−) – distinguishing feature from T cells.
    • Overall: NK cells are CD56+ CD16+ CD3− TCR−.
  • Function: rapid innate cytotoxicity against virus-infected and tumor cells; do not require prior sensitization or MHC recognition for activity.
  • Mechanisms of Action:
    • Perforin + granzymes induce apoptosis in target cells.
    • Fas–FasL interactions also trigger apoptosis.
    • Cytokine production (IFN-γ, TNF-α) enhances macrophage and adaptive responses.
  • Recognition balance:
    • Inhibitory receptors (e.g., KIR, CD94/NKG2A) recognize MHC class I to prevent killing of normal cells.
    • Activating receptors (e.g., NKG2D) recognize stress-induced ligands on infected or tumor cells to promote killing.
    • If MHC I is downregulated (common in viral infections and tumors) NK cells activate and kill.
  • Clinical note: NK cells constitute roughly 5%−20%5\%-20\% of circulating lymphocytes and are considered a distinct third type of lymphocyte.

WBC Diapedesis, Movement, and Phagocytosis – Key Concepts

  • Diapedesis: ability of WBCs to squeeze through narrow postcapillary venules to reach tissues.
  • Ameboid movement: neutrophils, monocytes, and lymphocytes exhibit ameboid movement, involving cytoplasmic extension and shape change.
  • Chemotaxis: directed migration toward injured tissue via chemical cues.
  • Phagocytosis: ingestion and digestion of microbial invaders; primarily neutrophils and monocytes/macrophages.
  • Overall: WBCs coordinate to defend against infection and maintain tissue homeostasis; each lineage contributes uniquely.

Summary: Key Features by Cell Type (at a glance)

  • Neutrophils (PMNs): 60%−70%60\%-70\% of WBCs; multilo and pale granules; first responders; phagocytosis; MPO-mediated killing; NETs; Barr body in females; left shift; toxic granulation and Döhle bodies; NADPH oxidase and CGD relevance.
  • Eosinophils: 1%−4%1\%-4\%; bilobed nucleus; strong red/orange granules; MBP and other toxic granule contents; defense against parasites; modulate allergic responses; histology: bright red-orange cytoplasm.
  • Basophils: <1%1\%; bilobed nucleus obscured by dark granules; histamine, heparin, leukotrienes; mediates Type I hypersensitivity; FcεRI IgE receptor.
  • Monocytes: 2%−8%2\%-8\%; largest WBC; kidney-shaped nucleus; differentiate into macrophages; phagocytosis; antigen presentation (MHC II); cytokines (IL-1, TNF-α).
  • Lymphocytes: 20%−40%20\%-40\%; small vs large lymphocytes; T cells (cell-mediated), B cells (humoral), NK cells (innate cytotoxicity); markers: CD3 (T), CD19/20 (B), CD16/56 (NK).
  • Plasma Cells: derived from B cells; eccentric nucleus with clock-face chromatin; basophilic cytoplasm; prominent Golgi; antibody production (IgM, IgG, IgA, IgE); lifespan 10–30 days; tissue residency for long-lived plasma cells.
  • NK Cells: large granular lymphocytes; CD56+, CD16+; CD3−, TCR−; innate cytotoxicity; perforin/granzymes; FasL; IFN-γ/TNF-α production; kill virus-infected and tumor cells without prior sensitization.

Notable Histological & Molecular Details to Remember

  • Barr body: drumstick-shaped projection indicating inactive X chromosome; seen in female neutrophils.
  • Döhle bodies: blue inclusions representing remnants of rough endoplasmic reticulum.
  • Myeloperoxidase (MPO): key enzyme in neutrophil microbicidal activity; catalyzes HOCl production via the reaction shown above.
  • MBP (Major Basic Protein): major toxic component of eosinophil granules; essential for parasite killing.
  • FcεRI: high-affinity IgE receptor on basophils and mast cells; cross-linking triggers degranulation and mediator release.
  • Antigen presentation: monocytes/macrophages express MHC II to present to helper T cells; B cells present to T cells in germinal centers via MHC II; CD1 molecules present lipid antigens.
  • Plasma cell clock-face nucleus and perinuclear Golgi hof are hallmark histology features.
  • NK cell markers: CD56 and CD16; lack CD3/TCR differentiates from T cells; ADCC via CD16 engages antibody-coated targets.

Equations & Important Formulas

  • MPO-catalyzed respiratory burst product:
    H<em>2O</em>2+Cl−→MPOHOCl+H2O.\mathrm{H<em>2O</em>2} + \mathrm{Cl^-} \xrightarrow{\text{MPO}} \mathrm{HOCl} + \mathrm{H_2O}.
  • Note: The respiratory burst involves NADPH oxidase assembly and generation of reactive oxygen species (not a single equation here, but MPO reaction is central).

Connections to Broader Immunology & Clinical Relevance

  • Neutrophils are essential for early bacterial clearance; defects in their function lead to susceptibility to bacterial infections (e.g., CGD).
  • Eosinophils serve as key effector cells against helminth infections and modulate allergic inflammation; excessive eosinophilia is seen in parasitic infections and some allergic diseases.
  • Basophils share functional overlap with mast cells; IgE-driven degranulation drives immediate hypersensitivity reactions.
  • Monocytes/macrophages bridge innate and adaptive immunity via phagocytosis and antigen presentation; dysregulated macrophage activation contributes to chronic inflammation and sepsis.
  • Lymphocytes (T, B, NK) coordinate adaptive immune responses; plasma cells are the antibody factories that provide targeted humoral immunity; NK cells provide rapid innate cytotoxicity without prior antigen exposure.
  • The balance and interaction among these cell types underlie normal immune defense and are perturbed in autoimmune diseases, immunodeficiencies, and various infections.