Histology of blood and hematopoiesis

White Blood Cells (WBCs) - The Immune Patrol

• Granulocytes (specific granules): Neutrophils, Eosinophils, Basophils.
• Agranulocytes (no specific granules): Lymphocytes, Monocytes.

📌 Mnemonic (WBC abundance): Never Let Monkeys Eat Bananas (Neutrophils > Lymphocytes > Monocytes > Eosinophils > Basophils).

• Neutrophils (40-60%): Multi-lobed nucleus; phagocytose bacteria.
• Lymphocytes (20-40%): Large, round nucleus; adaptive immunity (B/T cells).
• Monocytes (2-8%): Kidney-shaped nucleus; become macrophages.
• Eosinophils (1-4%): Bilobed nucleus, eosinophilic granules; fight parasites, allergic reactions.
• Basophils (<1%): Bilobed nucleus, basophilic granules; release histamine.

⭐ Left Shift: An increase in band neutrophils (immature forms) signals an acute bacterial infection, as the bone marrow rapidly releases them into circulation.

Hematopoiesis - The Blood Cell Factory

• Sites of Hematopoiesis Timeline:
  • Yolk Sac (3-8 wks)
  • Liver (6 wks-birth)
  • Spleen (10-28 wks)
  • Bone Marrow (18 wks to adult)
  • 📌 Mnemonic: Young Liver Synthesizes Blood.

⭐ In adults, hematopoiesis is primarily in the red marrow of the axial skeleton (vertebrae, sternum, ribs, pelvis) and proximal ends of long bones. Yellow marrow is inactive but can be reactivated under stress.

High‑Yield Points - ⚡ Biggest Takeaways

• Erythrocytes are anucleated, biconcave discs with a 120-day lifespan for O₂ transport.
• Neutrophils, the most abundant WBCs, have multi-lobed nuclei and are central to acute inflammation.
• Eosinophils feature a bilobed nucleus and cytoplasmic granules; they combat parasitic infections.
• Basophils and mast cells release histamine and heparin, mediating Type I hypersensitivity.
• Platelets are anucleated fragments of megakaryocytes essential for primary hemostasis.
• Adult hematopoiesis occurs primarily in the axial skeleton (vertebrae, sternum, pelvis).