24. Leukocytes (WBCs): Number, Functions, Control of Leukopoiesis, and Immunity

I. Leukocytes: Types and Numbers

• Total WBC count: 3.5–10.5 × 10⁹/L

1. Granulocytes (with granules)

• Neutrophils:
  • Total: 2.2–6.5 × 10⁹/L
  • Banded (immature): 0.0–0.50 × 10⁹/L
  • Segmented (mature): 2.10–6.40 × 10⁹/L
  • Function: Phagocytosis, bacterial killing (oxidative burst), interferon secretion

• Eosinophils:
  • 0–0.13 × 10⁹/L
  • Function: Attack parasites, modulate allergic responses

• Basophils:
  • 0.00–0.5 × 10⁹/L
  • Function: Release histamine and heparin; mediate allergic reactions and asthma

2. Agranulocytes (without granules)

• Monocytes:
  • 0.1–1.0 × 10⁹/L
  • Function: Phagocytosis, antigen presentation

• Lymphocytes (T and B cells):
  • 1.3–3.9 × 10⁹/L
  • T cells: Cell-mediated immunity

  • T helper: Activate B cells and macrophages

  • Cytotoxic T cells: Kill infected cells via apoptosis
    • B cells: Produce antibodies (humoral immunity)

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take a NEBL

II. Leukocyte Differential Count (% in WBC population)

Type	Normal Range (%)
Banded neutrophils	0–6%
Segmented neutrophils	42–70%
Eosinophils	0–4%
Basophils	0–2%
Lymphocytes	22–48%
Monocytes	1–14%

CONTROL OF LEUKOPOIESIS

CONTROL OF LEUKOPOIESIS

=Form of haematopoiesis in which WBCs are formed in bone marrow located in bones in adults and haematopoetic organs in foetus.
-WBCs are formed from the differentiation of pluripotent hematopoietic stem cells

1. Myeloid stem cells differentiate intoàRBCs, platelets (thrombopoietin in the liver), granulocytes, and monocytes

2. Lymphoid stem cells differentiate intoàlymphocytes

REGULATION FOR FORMATION OF GRANULOCYTES: -Interleukin 3,5
-Granulocytic colony stimulating factors

REGULATION FOR FORMATION OF AGRANULOCYTES: -Interleukin 3,5

IV. Immunity

Innate Immunity (Natural / Non-specific)

• Fast acting

• Physical and chemical barriers:

  • Reflexes (sneezing), phagocytosis, clotting

  • Biological barriers: skin, stomach acid, respiratory mucus

Adaptive Immunity (Acquired / Specific)

• Slower, but forms memory

• Two types:

  • Humoral: B cells → antibodies

  • Cell-mediated: T cells → targeted cell killing

Types of Acquired Immunity

Type	Description	Duration
Active	Natural (infection) or artificial (vaccination)	Long-term
Passive	Natural (placenta, breast milk) or artificial (antibody injection)	Short-term

V. Platelets (Thrombocytes)

• Count: 140–400 × 10⁹/L

• Mean Platelet Volume (MPV): 7.8–11 fL

• Origin: Derived from megakaryocytes which fragment before entering circulation

• Function: Plug damaged vessels and assist in clotting