17.3 Cellular Defenses

Blood is the body's cellular defense superhero team. It's made up of plasma, the liquid carrier, and formed elements like red blood cells, white blood cells, and platelets. Each plays a crucial role in keeping us healthy.

Hematopoiesis is the process that creates these cellular defenders. It happens in bone marrow, where stem cells give rise to all blood cell types. This constant production is key to maintaining a strong immune system and fighting off invaders.

Components and Formation of Blood in Cellular Defense

Components of blood for defense

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• Plasma
  • Liquid component of blood contains dissolved substances (proteins, nutrients, waste products)
  • Transports cells and molecules involved in cellular defense enables efficient distribution throughout the body
• Formed elements
  • Red blood cells (erythrocytes) deliver oxygen to tissues support immune cell function but not directly involved in defense
  • White blood cells (leukocytes) primary cells involved in immune response and cellular defense
    • Types include neutrophils, eosinophils, basophils, monocytes, lymphocytes each with specialized roles
  • Platelets (thrombocytes) involved in blood clotting and wound healing (hemostasis)
    • Release cytokines and growth factors contribute to inflammation and immune response modulation

Process of hematopoiesis

• Hematopoiesis formation and development of blood cells occurs primarily in bone marrow in adults
• Hematopoietic stem cells (HSCs) give rise to all blood cell types through differentiation process
  • HSCs differentiate into myeloid and lymphoid progenitor cells
    1. Myeloid progenitors develop into red blood cells, platelets, most white blood cells (except lymphocytes)
    2. Lymphoid progenitors develop into lymphocytes (B cells, T cells, NK cells)
• Maintaining steady supply of blood cells crucial for immune function
  • White blood cells have limited lifespans must be continually replenished via hematopoiesis
  • Insufficient hematopoiesis can lead to immunodeficiencies impaired cellular defense increased susceptibility to infections

Formed elements in innate immunity

• Neutrophils
  • Most abundant white blood cells in circulation first responders to infection
  • Phagocytose and destroy invading pathogens (bacteria, fungi)
  • Release antimicrobial proteins and reactive oxygen species kill pathogens
• Eosinophils
  • Defend against parasitic infections (helminths)
  • Release cytotoxic granules damage parasites
  • Modulate inflammatory responses regulate immune response
• Basophils
  • Release histamine and other inflammatory mediators promote vasodilation and immune cell recruitment
  • Involved in allergic reactions and defense against parasites mediate hypersensitivity responses
• Monocytes
  • Circulate in blood migrate into tissues differentiate into macrophages and dendritic cells
  • Phagocytose pathogens and cellular debris clear infections and maintain tissue homeostasis
  • Present antigens to lymphocytes initiate adaptive immune responses link innate and adaptive immunity
• Natural killer (NK) cells
  • Recognize and kill virus-infected and tumor cells through cytotoxic mechanisms
  • Release cytotoxic granules and induce apoptosis in target cells eliminate abnormal cells
  • Produce cytokines (interferon-γ) regulate immune responses modulate other immune cell functions

Immune System Overview

• Innate immunity: First line of defense, provides rapid, non-specific protection against pathogens
  • Includes physical barriers, cellular components, and soluble factors
  • Initiates inflammation to contain and eliminate threats
• Adaptive immunity: Develops over time, provides specific and long-lasting protection
  • Involves B and T lymphocytes, which recognize specific antigens
  • Generates immunological memory for faster future responses
• Complement system: Enhances both innate and adaptive immune responses
  • Consists of plasma proteins that work together to opsonize pathogens and promote their clearance
  • Activates inflammation and recruits immune cells to sites of infection