18.4 Leukocytes and Platelets

Leukocytes, or white blood cells, are the body's defense squad. They fight off invaders like bacteria and viruses, keeping us healthy. Each type of leukocyte has a unique role in the immune system, from neutrophils that gobble up pathogens to lymphocytes that remember past infections.

Platelets are tiny cell fragments that spring into action when we're injured. They stick together to form clots, stopping bleeding and helping wounds heal. Understanding how leukocytes and platelets work is key to grasping how our bodies protect themselves and repair damage.

Leukocytes

Characteristics and functions of leukocytes

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• Leukocytes, or white blood cells (WBCs), are essential components of the immune system that defend the body against infectious agents (bacteria, viruses) and foreign substances
• Leukocytes can exit the bloodstream and migrate to tissues where they are needed to fight infections or inflammation
• Leukocytes are categorized into granulocytes (neutrophils, eosinophils, basophils) which contain granules in their cytoplasm, and agranulocytes (lymphocytes, monocytes) which lack granules
• Leukocytes are produced in the bone marrow through hematopoiesis, where they originate from hematopoietic stem cells and differentiate into specific types based on the presence of cytokines and growth factors

Types of leukocytes in immune response

• Neutrophils (50-70% of leukocytes)
  • Possess a multilobed nucleus and fine, pale pink granules in the cytoplasm
  • Primary role is phagocytosis of bacteria and fungi, serving as the first responders to sites of infection or inflammation
• Eosinophils (2-4% of leukocytes)
  • Have a bilobed nucleus and large, coarse, reddish-orange granules in the cytoplasm
  • Involved in allergic reactions, defense against parasites, and release enzymes that neutralize inflammatory chemicals
• Basophils (0.5-1% of leukocytes)
  • Contain a lobed nucleus and large, dark blue-purple granules in the cytoplasm
  • Release histamine and other inflammatory mediators during allergic reactions, contributing to the inflammatory response
• Lymphocytes (25-35% of leukocytes)
  • Characterized by a large, round nucleus with a thin rim of cytoplasm
  • Two main types: T lymphocytes for cell-mediated immunity (direct attack on infected or cancerous cells) and B lymphocytes for humoral immunity (antibody production)
  • Play a crucial role in recognizing and responding to specific antigens
• Monocytes (3-8% of leukocytes)
  • Feature a large, kidney-shaped nucleus and abundant pale-blue cytoplasm
  • Migrate into tissues and differentiate into macrophages and dendritic cells
  • Phagocytose pathogens, cellular debris, and foreign substances, and present antigens to lymphocytes to initiate specific immune responses

Immune System and Inflammation

• The immune system is a complex network of cells, tissues, and organs that work together to defend the body against pathogens and foreign substances
• Inflammation is a protective response of the immune system to injury or infection, characterized by redness, swelling, heat, and pain
• Bone marrow plays a crucial role in the immune system by producing and maturing various types of immune cells

Platelets and Hemostasis

Structure and function of platelets

• Platelets (thrombocytes) are small, disc-shaped cell fragments derived from megakaryocytes in the bone marrow through a process of fragmentation
• Platelets lack a nucleus but contain organelles, granules, and enzymes essential for their function in hemostasis (stopping bleeding and repairing damaged blood vessels)
• Hemostasis involves three main stages:

1. Vascular spasm
2. Platelet plug formation
3. Coagulation (blood clotting)

• When a blood vessel is injured, platelets adhere to the damaged endothelium, become activated, and release granules containing clotting factors and growth factors
• Activated platelets change shape and aggregate to form a platelet plug, sealing the damaged vessel
• Platelets also contribute to the coagulation cascade by providing a surface for the assembly of clotting factors which convert fibrinogen into fibrin, forming a mesh-like network that stabilizes the platelet plug and creates a solid clot

Disorders of leukocytes and platelets

• Leukocyte disorders:
  • Leukopenia: low leukocyte count, increases susceptibility to infections, caused by medications, autoimmune disorders, or bone marrow disorders
  • Leukocytosis: high leukocyte count, often a sign of infection, inflammation, or leukemia, caused by stress, smoking, or certain medications
  • Leukemia: blood cancer, uncontrolled production of abnormal leukocytes, impairs normal blood cell production and function, leads to anemia, infections, and bleeding disorders
• Platelet disorders:
  • Thrombocytopenia: low platelet count, increases risk of bleeding and impairs clotting, caused by autoimmune disorders, medications, or bone marrow disorders
  • Thrombocytosis: high platelet count, increases risk of blood clots and thrombosis, caused by chronic inflammation, iron deficiency, or certain cancers
  • Platelet dysfunction: impaired platelet function despite normal count, can be inherited or acquired, leads to prolonged bleeding time and impaired clot formation, caused by medications (aspirin, NSAIDs)