Lecture 20: Host Defense System and Immunology

Summary of Host Defense System

Three Lines of Host Defense System
First Line of Defense: Anatomical Barriers

  • Skin: Outermost layer of the body.

    • Composed of keratin.

    • Contains hair follicles and skin glands (sebaceous secretions).

  • Mucous Membrane: Present in various systems.

    • Digestive System: Protects against pathogens ingested with food.

    • Urinary System: Protects against infections via urinary tract.

    • Respiratory System: Trachea and respiratory tree contain cilia that entrap and propel particles out of the respiratory tract.

    • Eye: Mucous membranes help maintain moisture and protection.

  • Sebaceous Glands: Secrete sebum, an oil that can inhibit microbial growth.

  • Tears and Saliva: Contain lysozyme, an enzyme that hydrolyzes peptidoglycan, leading to bacterial cell lysis.

  • pH levels:

    • Acidic pH: Found in sweat, stomach, skin, and vagina, which helps inhibit microbial survival.

    • Basic pH: Found in semen, which also plays a role in microbial control.

First Line of Defense: Chemical Barriers

  1. Unbroken Skin: Physical barrier to pathogens.

  2. Cilia in Respiratory Tract: Traps and moves pathogens out of the respiratory system.

  3. Lysozyme in Saliva: Enzymatic breakdown of bacterial cell walls.

  4. All of These: Highlighting the importance of all components.

  5. None of These: Incorrect option.

Second and Third Lines of Defense

  • Immunology: Study of the body's resistance to infectious agents.

  • Immune System Functions:

    • Surveillance of the body for foreign material.

    • Recognition of foreign substances.

    • Destruction of foreign agents.

  • Involvement of both nonspecific and specific immune defense systems.

  • White Blood Cells (WBC): Key players in the immune response.

White Blood Cells (WBCs)

  • Self-Markers:

    • Proteins or sugars on host cells that allow WBCs to recognize self vs. non-self.

    • Autoimmune disorders occur when there is a failure to recognize self markers.

  • Non-self Markers: Recognized as foreign, prompting an immune response.

    • PAMPs: Pathogen Associated Molecular Patterns that are recognized by immune cells.

Tissues, Organs, and Cells of the Immune System

  • System Integration: Various components work together for immunity.

  • Components include:

    • Reticuloendothelial System (RES): Also known as the Mononuclear Phagocyte System (MPS).

    • Contains phagocytic cells in connective tissue networks, allowing immune cells to traverse outside blood and lymphatic systems.

    • Extracellular Fluids System (ECF): Spaces surrounding tissue cells enabling immune cells to move freely.

    • Blood Circulatory System: Supplies immune cells to tissues.

    • Lymphatic System: Aids in the return of extracellular fluid and provides immune defense features.

Reticuloendothelial System (MPS)

  • Phagocytic Cells: High presence of macrophages within connective tissue.

  • Method of Action: Facilitates intercommunication among immune cells outside blood and lymph.

Blood Composition

  • Whole Blood: Blood cells suspended in plasma (92% water, other nutrients, hormones, clotting factors, immunochemicals, ions, and waste products).

  • Stem Cells: Precursor cells involved in the production of blood components.

  • Hematopoiesis:

    • Definition: The production of blood cells, starting in the embryonic stage and continuing in adults via the bone marrow.

Types of Blood Cells

  • White Blood Cells (WBC): Include granulocytes and agranulocytes.

    • Granulocytes:

    1. Neutrophils: 55-90% of WBCs; polymorphonuclear; first responders during inflammation; phagocytize bacteria using digestive enzymes.

    2. Eosinophils: 1-3% of WBCs; bilobed nucleus; involved in attacking eukaryotic pathogens; associated with inflammation and allergies.

    3. Basophils: <0.5% of WBCs; function similar to eosinophils; known for releasing mediators during allergic responses.

    • Agranulocytes:

    1. Monocytes: 3-7% of WBCs; largest WBC; differentiate into macrophages or dendritic cells for phagocytosis.

    2. Lymphocytes: 20-35% of WBCs; responsible for specific immunity (B cells, T cells, natural killer cells).

Mnemonic for WBCs

  • Never Let Monkeys Eat Bananas:

    • Neutrophils (most common)

    • Lymphocytes

    • Monocytes

    • Eosinophils

    • Basophils (least common)

Other Blood Cell Types

  • Red Blood Cells (Erythrocytes):

    • No nuclei; transport oxygen and carbon dioxide; most numerous circulating blood cells.

  • Platelets:

    • Fragments of cells, responsible for hemostasis (clotting) and tagging bacteria for immune response.

Lymphatic System

  • Definition: A network of vessels extending throughout the body; auxiliary route for extracellular fluid return to circulatory system.

  • Functions:

    1. Provides an auxiliary route for the return of extracellular fluid to circulation.

    2. Acts as a “drain-off” for inflammatory response.

    3. Facilitates surveillance, recognition, and protection via lymphocytes, phagocytes, and antibodies.

Lymphatic Fluids

  • Composition: Plasma-like fluid composed of water, dissolved salts, proteins (e.g. antibodies, albumin), and white blood cells (lymphocytes); no red blood cells.

  • Source: Formed from blood components diffusing into lymphatic capillaries.

Lymphatic Vessels

  • Parallels the blood system, thin-walled.

  • Transports lymph back to the bloodstream; lymph movement depends on muscle contractions.

Lymph Nodes

  • Clusters located throughout the body; filter lymph and provide an environment for immune reactions.

Spleen

  • Located in the upper left abdomen; filters blood, traps pathogens, and phagocytizes them; important for immune surveillance.

  • Adults can survive without a spleen, but asplenic children are severely immunocompromised.

Thymus

  • Two lobes in embryo; responsible for T cell maturation; decreases in size after puberty.

  • Major site for T cell differentiation.

Other Lymphoid Tissues

  • Tonsils: Collection of lymphoid tissues with lymphocytes.

  • Skin-associated lymphoid tissue (SALT), Mucosa-associated lymphoid tissue (MALT), and Gut-associated lymphoid tissue (GALT) (e.g., appendix, lacteals, Peyer’s patches).

Second Line of Defense: Non-specific Immunity

  • Components:

    • Inflammation (includes fever)

    • Phagocytosis

    • Antimicrobial Proteins (e.g., Interferon, Complement, Antimicrobial peptides)

    • Restriction factors

Inflammation

  • Major Response to Tissue Trauma:

    • Symptoms include:

    1. Rubor (redness)

    2. Calor (warmth/heat)

    3. Tumor (swelling)

    4. Dolor (pain)

    5. Loss of function.

Causes of Inflammation

  • Trauma, tissue injury, or specific immune reactions.

Functions of Inflammation

  • Mobilizes and attracts immune components to injury sites; aids tissue repair; localizes and removes harmful substances; destroys microbes to block invasion.

Fever

  • Normal body temperature regulated by the hypothalamus.

  • Induced by pyrogens:

    • Exogenous pyrogens originate externally (e.g., from infectious agents).

    • Endogenous pyrogens originate internally (e.g., products from leukocytes).

  • Fever: Inhibits microbial and viral multiplication, reduces nutrient availability for pathogens, increases immune reactions.