White Blood Cells (Leukocytes)

Group 1: Granulocytes

1. Neutrophils

2. Eosinophils

3. Basophils

Group 2: Agranulocytes

4. Lymphocytes

5. Monocytes

Key Concept: Diapedesis

Quick Summary Table

White Blood Cells (WBCs / Leukocytes) — Physiology, Types & Functions

Genesis of WBCs (Leukopoiesis)

Classification & Normal Differential Count

Group 1: Granulocytes (Polymorphonuclear Leukocytes / PMNs)

1. 🔵 Neutrophils (62%)

• First responders to bacterial infection — arrive at infection sites within minutes
• Perform phagocytosis: engulf and digest bacteria, fungi, debris
• Move by ameboid motion at up to 40 µm/min through tissue
• Exit blood vessels by diapedesis (extravasation) — squeezing through endothelial gaps
• Are guided to infection sites by chemotaxis — respond to chemical signals up to 100 µm away (bacterial toxins, complement fragments, tissue breakdown products)
• Contain myeloperoxidase, lactoferrin, defensins, and elastase in granules for killing pathogens
• Release neutrophil extracellular traps (NETs) — chromatin webs that trap/kill bacteria
• Lifespan: Only 1–4 days in tissues; billions die by apoptosis daily

2. 🔴 Eosinophils (2.3%)

• Primary defense against helminthic (worm) parasites — release toxic granule contents onto parasite surfaces
• Important in allergic reactions and asthma — migrate to sites of IgE-mediated reactions
• Modulate inflammation — produce prostaglandins, leukotrienes, cytokines
• Phagocytose antigen-antibody complexes
• Lifespan: 1–2 weeks in tissues

3. 🟣 Basophils (0.4%)

• Release histamine → vasodilation and increased vascular permeability during allergic reactions
• Release heparin → prevents blood clotting locally and helps transport fat particles
• Release serotonin, leukotrienes (slow-reacting substances of anaphylaxis) → bronchoconstriction
• Bear IgE receptors on their surface → activated in Type I hypersensitivity (anaphylaxis, asthma)
• Functionally similar to mast cells in connective tissue
• Lifespan: 1–3 days

Group 2: Agranulocytes

4. 🟤 Lymphocytes (30%)

• Memory cells formed after first exposure → faster response on re-exposure (basis of vaccination)
• Lifespan: Hours to many years (memory cells survive decades)

5. 🟠 Monocytes (5.3%)

• Circulate in blood for 10–20 hours, then enter tissues and differentiate into macrophages (diameter increases 5-fold to 60–80 µm)
• Macrophages are extremely potent phagocytes — the tissue-based second line of defense

Key Physiological Mechanisms

Diapedesis & Chemotaxis

1. Margination — WBCs adhere to vessel endothelium near infection site (P-selectin/ligand interactions)
2. Diapedesis — WBCs squeeze through endothelial gaps into tissue
3. Chemotaxis — WBCs migrate toward infection guided by chemical gradients (bacterial toxins, complement products, cytokines like IL-1, TNF-α)
4. Phagocytosis — engulf and destroy pathogens using reactive oxygen species, enzymes, and antimicrobial proteins
5. Opsonization — antibodies (IgG) and complement (C3b) coat bacteria, making them easier to phagocytose

Lifespan of WBCs

Clinical Significance

Mechanism of Pathogenesis

"Individuals are sick because their cells are sick."

The Four Pillars of Disease (Pathology Framework)

Step 1 — Etiology (The Initiating Cause)

A. Genetic Causes

• Inherited mutations (e.g., cystic fibrosis gene CFTR)
• Acquired mutations (somatic mutations in cancer)
• Gene variants/polymorphisms that increase susceptibility

B. Environmental Causes

• Infectious agents - bacteria, viruses, parasites, fungi, prions
• Physical agents - radiation, trauma, extreme temperature, electricity
• Chemical agents - drugs, toxins, alcohol, cigarette smoke, pollutants
• Nutritional deficiencies - vitamins, proteins, minerals
• Immunologic factors - autoimmunity, hypersensitivity reactions

Most common diseases (cancer, atherosclerosis, diabetes) are multifactorial - they arise from environmental insults acting on a genetically susceptible individual.

Step 2 — Cellular Response to Stress (The Core Pathogenic Mechanism)

2A. Cellular Adaptation (Reversible - Cell Survives)

2B. Reversible Cell Injury

• Cellular swelling (most common early sign)
• Fatty change (lipid accumulation in cells)
• Plasma membrane blebbing and loss of microvilli
• Mitochondrial swelling
• Dilation of the endoplasmic reticulum
• Clumping of nuclear chromatin

2C. Irreversible Cell Injury → Cell Death

Step 3 — Two Pathways of Cell Death

🔴 Pathway 1: NECROSIS (Pathological Death)

1. Severe injury → membrane disruption
2. Intracellular enzymes and proteins leak out into surrounding tissue
3. Leaked contents trigger inflammatory response (redness, swelling, pain)
4. Macrophages and neutrophils arrive to clean up debris

• Eosinophilic (pink) cytoplasm on H&E staining (due to loss of RNA)
• Nuclear changes: pyknosis (shrinkage) → karyorrhexis (fragmentation) → karyolysis (dissolution)
• Breakdown of plasma and organelle membranes
• Myelin figures (whorled phospholipid masses)

🟢 Pathway 2: APOPTOSIS (Programmed Death)

1. Cell receives a "death signal" (intrinsic or extrinsic pathway)
2. Caspase enzymes are activated (the executioner proteins)
3. DNA is fragmented by DNAses into a characteristic "ladder" pattern
4. Cell shrinks and breaks into apoptotic bodies (membrane-bound fragments)
5. Phagocytes recognize "eat me" signals (phosphatidylserine exposure) and engulf them
6. No inflammatory response - clean, silent death

• Development (sculpting of fingers, removal of excess neurons)
• Hormone-driven involution (uterine lining during menstruation)
• Elimination of self-reactive lymphocytes (immune tolerance)
• Removal of aged/damaged cells (neutrophils after acute infection resolves)

• Virus-infected cells
• DNA-damaged cells (tumor suppressor p53 activates apoptosis)
• Ischemic injury (e.g., brain after stroke)

Step 4 — Downstream Pathogenic Mechanisms

🔥 Inflammation

• Necrotic cells release damage-associated molecular patterns (DAMPs)
• Macrophages release cytokines (IL-1, TNF-α, IL-6)
• Neutrophils are recruited → acute inflammation
• If unresolved → chronic inflammation → fibrosis (scarring)
• Chronic inflammation is the pathogenic basis of atherosclerosis, rheumatoid arthritis, IBD, and many cancers

🧬 Genetic/Molecular Dysregulation

• Mutations in proto-oncogenes → oncogenes → uncontrolled cell proliferation → cancer
• Loss of tumor suppressor genes (p53, Rb) → failure of cell cycle control
• Epigenetic changes → gene silencing or overexpression

🛡️ Immune-Mediated Injury

• Type I hypersensitivity (IgE/mast cells): Anaphylaxis, asthma
• Type II (antibody-mediated): Autoimmune hemolytic anemia
• Type III (immune complex): SLE, serum sickness
• Type IV (T cell-mediated): Contact dermatitis, graft rejection, tuberculosis

🧪 Accumulations & Deposits

• Lipid accumulation → fatty liver → cirrhosis
• Abnormal protein folding → amyloidosis
• Calcium deposition → dystrophic calcification
• Glycogen/lipid storage in lysosomal storage disorders

Summary Flowchart

ETIOLOGY (Cause)
    ↓
Genetic / Environmental Insult
    ↓
Cellular Stress
    ↓  ↙ Mild stress     → ADAPTATION (hypertrophy, hyperplasia, atrophy, metaplasia)
    ↓  ↙ Mild/transient  → REVERSIBLE INJURY (swelling, fatty change)
    ↓  ↙ Severe/sustained → IRREVERSIBLE INJURY
         ↙             ↘
    NECROSIS        APOPTOSIS
  (uncontrolled,   (controlled,
   inflammatory)    non-inflammatory)
         ↓
   Morphological Changes (tissue damage, fibrosis, organ dysfunction)
         ↓
   CLINICAL MANIFESTATIONS (symptoms, signs, disease)