Differential Leucocyte Count (DLC) - MBBS 1st Year

Definition

Classification of Leucocytes

• Neutrophils
• Eosinophils
• Basophils

• Lymphocytes
• Monocytes

Normal DLC Values (Adults)

Memory mnemonic: "Never Let Monkeys Eat Bananas" (N-L-M-E-B in decreasing order)

Each Cell in Detail

1. Neutrophils (50-70%)

• Also called: Polymorphonuclear leucocytes (PMNs), Segmented neutrophils, Segs
• Size: 10-12 µm
• Nucleus: 2-5 lobes connected by thin chromatin filaments (multilobed/segmented)
• Granules:
  • Azurophilic (primary) granules: Contain myeloperoxidase, lysozyme, defensins, elastase
  • Specific (secondary) granules: Contain lactoferrin, collagenase, alkaline phosphatase
  • Tertiary (gelatinase) granules: Contain gelatinase (MMP-9)
• Life span: Circulate 8-12 hours; live 1-2 days in connective tissue
• Function:
  • First line of defense against bacterial and fungal infections
  • Phagocytosis and intracellular killing via oxidative burst (respiratory burst)
  • Release of proteolytic enzymes and reactive oxygen species (ROS)
  • Key role in acute inflammation
• Increased in (Neutrophilia):
  • Bacterial infections
  • Acute inflammation, MI, burns, surgery
  • Corticosteroid therapy
  • Myeloproliferative disorders (CML)
• Decreased in (Neutropenia):
  • Viral infections
  • Aplastic anemia, chemotherapy
  • WHIM syndrome (CXCR4 mutation - retention in bone marrow)
  • Typhoid fever

2. Lymphocytes (20-40%)

• Size: 6-15 µm (small lymphocytes ~6-9 µm; large lymphocytes ~10-15 µm)
• Nucleus: Large, round to slightly indented, dark-staining (dense chromatin), occupies most of the cell
• Cytoplasm: Scant, pale blue
• Types:
  • T lymphocytes (T cells): 70-80% of circulating lymphocytes; cell-mediated immunity (CD4+ helper, CD8+ cytotoxic)
  • B lymphocytes (B cells): 10-15%; humoral immunity - differentiate into plasma cells that produce antibodies
  • NK cells (Natural Killer): 5-10%; kill virus-infected and tumor cells without prior sensitization
• Function: Adaptive (acquired) immunity - both humoral and cell-mediated
• Increased in (Lymphocytosis):
  • Viral infections (EBV - infectious mononucleosis, CMV, hepatitis)
  • Whooping cough (Bordetella pertussis)
  • Chronic lymphocytic leukemia (CLL)
  • Physiological lymphocytosis in children (lymphocytes > neutrophils in infants up to ~4 years)
• Decreased in (Lymphopenia):
  • HIV/AIDS
  • Corticosteroid therapy
  • SLE, radiation

3. Monocytes (2-8%)

• Size: 15-20 µm (largest circulating leucocyte)
• Nucleus: Kidney-shaped, horseshoe, or folded (indented); less dense chromatin
• Cytoplasm: Abundant, gray-blue ("ground glass" appearance), contains fine azurophilic granules
• Precursor of: Tissue macrophages (Kupffer cells in liver, microglia in brain, osteoclasts in bone, alveolar macrophages in lung, Langerhans cells in skin)
• Circulate for: 1-3 days in blood, then migrate to tissues where they become macrophages and can live for months to years
• Function:
  • Phagocytosis of bacteria, dead cells, and cellular debris
  • Antigen presentation (to T cells via MHC II)
  • Secretion of cytokines (IL-1, IL-6, TNF-α)
  • Key role in chronic inflammation and granuloma formation
• Increased in (Monocytosis):
  • Chronic infections: TB, brucellosis, leishmaniasis
  • Infective endocarditis
  • Monocytic leukemia

4. Eosinophils (1-4%)

• Size: 10-15 µm
• Nucleus: Typically bilobed (2 lobes connected by a thread) - characteristic feature
• Granules: Large, refractile, bright orange-red/eosinophilic granules (acidophilic) - contain:
  • Major Basic Protein (MBP): Toxic to helminths
  • Eosinophil Cationic Protein (ECP)
  • Eosinophil Peroxidase
  • Histaminase (inactivates histamine)
• Function:
  • Defense against parasitic (helminth) infections - too large to phagocytose, so degranulate onto them (ADCC)
  • Modulate allergic responses - limit the allergic reaction by breaking down histamine and leukotrienes
  • Phagocytosis of antigen-antibody complexes
• Increased in (Eosinophilia) - mnemonic NAACP:
  • Neoplasia (Hodgkin's lymphoma, T-cell lymphoma)
  • Allergic/Atopic disorders (asthma, eczema, urticaria, hay fever)
  • Addison's disease (adrenal insufficiency)
  • Collagen vascular diseases / parasitic infections
  • Parasitic infections (helminths - not protozoa)
• Decreased in (Eosinopenia):
  • Corticosteroid excess (Cushing's, steroid therapy)
  • Acute bacterial infections, acute stress

5. Basophils (0-1%)

• Size: 10-12 µm
• Nucleus: Bilobed or trilobed, but obscured by large granules
• Granules: Large, numerous, deeply basophilic (blue-purple), stain with basic dyes; contain:
  • Histamine (vasoactive amine)
  • Heparin (anticoagulant)
  • Leukotrienes (LTC4, LTD4)
  • Serotonin
• Rarest WBC in circulation
• Function:
  • Mediate immediate hypersensitivity (Type I / allergic) reactions via IgE receptors (FcεRI) on their surface - release histamine and other mediators upon antigen cross-linking
  • Basophils are the circulating counterpart to mast cells (tissue-resident)
• Increased in (Basophilia):
  • Allergic reactions
  • CML (basophilia is a hallmark; >basophils suggest blast crisis)
  • Hypothyroidism
  • Viral infections (chickenpox, influenza)

Granulopoiesis (Development of Granulocytes)

• Myelocytes are the first stage to exhibit specific granules - this is when the neutrophil, eosinophil, and basophil lines can first be distinguished
• The metamyelocyte has a kidney-bean shaped nucleus and numerous specific granules (specific:azurophilic ratio ~2:1 in neutrophils)
• Granulopoiesis takes ~2 weeks: 1 week mitotic phase (myeloblast to late myelocyte), 1 week postmitotic differentiation (metamyelocyte to mature granulocyte)
• The bone marrow produces >10¹¹ neutrophils per day
• Bone marrow retains a large storage pool of mature neutrophils (for 4-6 days), released rapidly under infection/inflammation

Shift to Left vs. Right

Clinical Correlates for MBBS

Physiological Crossover in Children

• At birth: Neutrophils > Lymphocytes
• At ~4-6 days (1st crossover): Lymphocytes = Neutrophils
• From ~1 week to ~4-6 years: Lymphocytes > Neutrophils (physiological lymphocytosis)
• At ~4-6 years (2nd crossover): Neutrophils = Lymphocytes
• After ~6 years: Neutrophils > Lymphocytes (adult pattern)

Summary Table