Overview
This lecture covers the characteristics, classification, and functions of leukocytes (white blood cells) and platelets, including their roles in immunity, development, and common disorders.
Characteristics of Leukocytes
- Leukocytes (WBCs) are larger, less numerous than erythrocytes, and possess a nucleus and organelles.
- They defend against pathogens, mutated cells, and clean up debris.
- Leukocytes can exit blood vessels via diapedesis to reach tissues.
- Movement is directed by chemotaxis, where chemical signals attract leukocytes to injury or infection.
- Differential leukocyte counts are important for diagnosis.
Classification of Leukocytes
Granular Leukocytes
- Neutrophils are the most common, rapid responders, phagocytize bacteria, and have multi-lobed nuclei.
- Eosinophils are involved in combating parasites and allergies, stain red-orange, and contain antihistamines.
- Basophils are least common, intensify inflammation, release histamine and heparin, and stain dark blue.
Agranular Leukocytes
- Lymphocytes (B, T, and NK cells) provide immune responses; B cells make antibodies, T cells attack cells, NK cells target abnormal cells.
- Monocytes are large, differentiate into macrophages, and phagocytize pathogens and debris.
Lifecycle and Disorders of Leukocytes
- Leukocytes develop from stem cells in bone marrow; lymphocytes mature in lymphatic tissue.
- Leukopenia: low WBC count, increases infection risk.
- Leukocytosis: high WBC count, but cells may be nonfunctional.
- Leukemia: cancer with excess, often dysfunctional leukocytes; classified as myelocytic or lymphocytic.
- Lymphoma: cancers of B or T lymphocytes accumulating in tissues; may progress slowly or rapidly.
Platelets: Structure, Function, and Disorders
- Platelets are cell fragments from megakaryocytes, not whole cells, and play a key role in clotting (hemostasis).
- Each megakaryocyte releases 2000–3000 platelets.
- Platelets secrete growth factors for tissue repair and are stored in the spleen for rapid release.
- Thrombocytosis is excess platelets, raising clot risk; thrombocytopenia is low platelets, causing bleeding issues.
Key Terms & Definitions
- Leukocyte (WBC) — a white blood cell involved in immune defense.
- Diapedesis — movement of leukocytes out of the bloodstream into tissues.
- Chemotaxis — movement of cells toward a chemical signal.
- Granulocyte — leukocyte with visible cytoplasmic granules (neutrophils, eosinophils, basophils).
- Agranulocyte — leukocyte with few or no visible granules (lymphocytes, monocytes).
- Megakaryocyte — large bone marrow cell that produces platelets.
- Hemostasis — the process of stopping blood loss after vessel injury.
Action Items / Next Steps
- Practice identifying leukocytes on blood smears using University of Michigan Webscopes.
- Construct a tally table and classify 50–100 cells to assess for normal or abnormal blood smear percentages.
- Review the next chapter on lymphatic system and immunity for more details on lymphocyte functions.