Leukopoiesis and Leukocyte Types

Leukopoiesis Overview

• Definition: Process of producing white blood cells (leukocytes) that play a crucial role in the immune system.
• Related terms:
  • Leukopenia: A decrease in white blood cell production.
  • Leukocytosis: An increase in white blood cell production.

Leukocyte Characteristics

• Leukocytes are less numerous than red blood cells.
• Functions:
  • Defense against foreign invaders.
  • Involvement in phagocytosis (cellular ingestion of microbes).
  • Participation in inflammatory responses and immune defense.

Types of Leukocytes

• Divided into two main categories:
  • Granulocytes: Contain granules in their cytoplasm.
    • Types include:
      • Neutrophils
      • Eosinophils
      • Basophils
  • Agranulocytes: Lack visible granules.
    • Types include:
      • Monocytes
      • Macrophages (tissue-resident monocytes)
      • Lymphocytes
      • Plasma cells (activated lymphocytes)

Granulocyte Development

• Granulopoiesis: Production of granulocytes.
• Stages:
  • Myeloblast: Most immature form, large, rarely seen in peripheral blood.
  • Promyelocyte: Second stage, lighter blue cytoplasm with non-specific granules.
  • Myelocyte: Third stage, smaller, more specific granules begin to appear.
  • Metamyelocyte: Fourth stage, no further cell division occurs.

Neutrophils

• Most common type of white blood cell in dogs and cats, primarily involved in fighting infections.
• Neutrophilia: Increase in neutrophils; can be caused by factors like stress, inflammation, and infections.
• Neutropenia: Decrease in neutrophils; can result from long-term steroid use or viral infections.
• Banded Neutrophils: Immature form with a horseshoe-shaped nucleus.

Eosinophils

• Characterized by pink/orange granules; involved in allergic reactions and defense against parasites.
• Eosinophilia: Increase in eosinophils commonly associated with allergies or parasitic infections.
• Eosinopenia: Decrease in eosinophils due to increased corticosteroids.

Basophils

• Contain purple granules and are involved in allergic reactions.
• Rare in circulation; thought to be tied to mast cells in tissues.
• Basophilia: Increase in basophils often in relation to allergies.

Lymphocytes

• Major producers of antibodies and main type of white blood cell in ruminants.
• Types:
  • B-lymphocytes: Mature in bone marrow; develop specific receptors and can become plasma cells.
  • T-lymphocytes: Mature in thymus; important for cellular immunity and activating B-cells.
• Lymphocytosis: Increase in lymphocytes due to infections or excitement.
• Lymphopenia: Decrease often caused by steroid response or Cushing's disease.

Monocytes and Macrophages

• Monocytes are larger than granulocytes, with a kidney bean-shaped nucleus.
• Functions include phagocytosis and antigen processing.
• In tissues, they differentiate into macrophages.

Smudge Cells

• Result from excessive pressure during blood film preparation or fragile cells.
• Appear as pale nuclear remnants; few are benign, many indicate cell fragility.

White Blood Cell Response to Infections

• Bacterial Infections: Typically show leukocytosis with neutrophilia followed by potential left shift.
• Viral Infections: Initial leukocytosis transitioning to leukocytopenia.

Leukogram Evaluation

• Stress Leukogram: Characterized by neutrophilia and lymphopenia.
• Inflammatory Leukogram: Shows neutrophilia with left shift.
• Excitement Leukogram: Caused by epinephrine release, showing lymphocytosis and neutrophilia without left shift.

Platelets

• Function in clotting; classified as cellular fragments from megakaryocytes, aiding hemostasis.