Human Anatomy and Physiology: Lymphatic System and Immunity

Human Anatomy and Physiology - Chapter 20 Study Notes

The Lymphatic System and Immunity

Structure and Function of the Lymphatic System

  • Definition: The lymphatic system is a group of organs and tissues collaborating with the immune system to combat disease-causing cells and substances known as pathogens. The system also performs functions associated with fluid homeostasis.

  • Main components:

    • (1) Lymphatic Vessels: A system of blind-ended vessels that transport lymph.

    • (2) Lymphatic Tissue and Lymphoid Organs: Includes clusters of lymphoid follicles (e.g., tonsils), lymph nodes, spleen, and thymus.

Functions of the Lymphatic System

  • Regulation of Interstitial Fluid Volume:

    • Blood capillaries experience net filtration pressure favoring filtration, resulting in a loss of 2–4 liters of water from blood plasma to interstitial fluid daily.

    • Lymphatic vessels pick up interstitial fluid, returning it to the cardiovascular system. Once fluid enters the lymphatic vessels, it is termed lymph.

  • Absorption of Dietary Fats:

    • Breakdown products of dietary fats enter small lymphatic vessels in the small intestine and are eventually delivered to the blood through lymph.

  • Immune Functions:

    • Lymphoid organs filter pathogens from not only lymph but blood as well, housing leukocytes responsible for fighting infections.

Lymphatic Vessels and Lymph Circulation

  • Lymph Circulation Process:

    • Lymph-Collecting Vessels merge to form nine larger Lymph Trunks, which receive lymph from specified body regions:

    • Lumbar Trunks: Collect from lower limbs and pelvic area.

    • Intestinal Trunk: Collects fat-containing lymph from the small intestine.

    • Jugular Trunks: Collects from the head and neck.

    • Bronchomediastinal Trunks: Collects from the thoracic cavity.

    • Subclavian Trunks: Collects from the upper limbs.

    • Cisterna Chyli: An enlargement where the intestinal and lumbar trunks drain.

    • Thoracic Duct: Drains the left side of the body into the junction of the left internal jugular and left subclavian vein.

    • Right Lymphatic Duct: Drains the right upper side of the body into the junction of the right internal jugular and right subclavian vein.

Characteristics of Lymphatic Circulation

  • Low-Pressure Circuit: Lymphatic vessels function without a built-in pump. They use additional mechanisms to assist fluid transport against gravity, such as:

    • Lymphatic Valves: Prevent backflow of lymph.

    • Muscle Contraction: Often found between muscle groups, where the contraction pushes lymph toward the heart.

    • Smooth Muscle Contractions: In lymph-collecting vessels also contribute to lymph movement.

Diseases Associated with the Lymphatic System

  • Edema: Swelling caused by the accumulation of interstitial fluid, which can be triggered by various conditions including trauma and heart failure.

  • Lymphedema: Severe edema often resulting from surgical removal or blockage of lymphatic vessels, leading to tissue enlargement.

    • Example: Lymphedema in the arm of breast cancer patients post lymph node removal.

Structure of Lymphatic Tissue and Organs

  • Reticular Tissue: The predominant tissue in the lymphatic system, known as lymphoid tissue, characterized by thin reticular fibers forming nets that trap pathogens.

  • Cell Types:

    • Lymphocytes: Includes B cells (B lymphocytes) and T cells (T lymphocytes).

    • Phagocytes: Includes macrophages (mature monocytes) and dendritic cells, known for spiny processes.

    • Reticular Cells: Produce reticular fibers akin to specialized collagen proteins.

Mucosa-Associated Lymphatic Tissue (MALT)

  • Definition: Loosely organized clusters of lymphoid tissues protecting mucous membranes exposed to pathogens.

  • Locations: Found in the gastrointestinal tract, respiratory passages, and partially in the genitourinary tract.

    • Clusters contain primarily B and T cells, potentially encapsulated or non-encapsulated.

  • Spherical Clusters:

    • Lymphoid Follicles/Nodules: Composed mainly of B cells; can develop germinal centers during pathogen exposure when B cells are dividing.

  • Specific Examples:

    • Tonsils (groups including Pharyngeal, Palatine, and Lingual Tonsils) and Peyer’s Patches in the small intestinal ileum.

  • Appendix: Has a role in bacteria defense and risk of appendicitis if feces/materials block its passages.

Lymph Nodes

  • Structure: Small, bean-shaped clusters of lymphatic tissue located along lymphatic vessels.

    • Categorized based on location: axillary, cervical, inguinal, and mesenteric nodes.

    • Consist of an external connective tissue capsule enclosing reticular fibers with macrophages, lymphocytes, and dendritic cells.

    • The interior divided into an outer cortex (B cell-rich) and medulla (mostly T cells).

Spleen

  • Largest lymphoid organ located in the left upper quadrant of the abdominopelvic cavity.

  • Internal structure: Comprises two types of pulp:

    • Red Pulp: Contains macrophages that destroy old erythrocytes.

    • White Pulp: Contains leukocytes that filter pathogens from the blood, principally around central arteries.

Thymus

  • Characteristics: A small, lobed organ in the superior mediastinum producing functional T cells via hormone secretion.

  • Development: Peaks in size around ages 12-14 and begins to atrophy with age, replaced by fat.

  • T cell maturation: Involves the death of self-reactive T cells, leading to an immunocompetent population.

Overview of the Immune System

  • Three Lines of Defense Against Pathogens:

    • First Line (Surface Barriers): Includes cutaneous (skin) and mucous membranes.

    • Second Line (Innate Immunity): Involves cellular and protein responses.

    • Third Line (Adaptive Immunity): Antigen-specific responses.

Categories of Immunity

  • Innate (Nonspecific) Immunity:

    • Acts against all pathogens without specificity; responds within 12 hours.

  • Adaptive (Specific or Acquired) Immunity:

    • Characterized by specific responses to particular antigens, includes cell-mediated and antibody-mediated immunity.

    • Antibodies involved bind to specific antigens, taking days to respond after exposure.

    • Immunological Memory: The immune system retains memory of past antigen encounters for rapid future responses.

Surface Barriers

  • Main Types:

    • Skin: Multi-layered with keratin, resistant to physical stress.

    • Mucous Membranes: Line external passages; thinner than skin epithelium and lack keratin.

  • Secretions:

    • Sebum: Antimicrobial oil secreted by sebaceous glands.

    • Mucus: Traps pathogens; produced by mucous membranes, with cilia moving debris out of respiratory passages.

    • Stomach Acid: Kills ingested pathogens.

    • Defensins: Antimicrobial peptides damaging pathogen membranes.

Pathogen Evasion Mechanisms

  • Collagenase-Producing Bacteria: E.g. Clostridium perfringens which leads to Gas Gangrene by breaking down collagen.

  • Phagocytosis-Resistant Fungi: These fungi’s thick walls hinder ingestion by phagocytes. Examples include Blastomycosis and Cryptococcosis.

  • Acid-Tolerant Pathogens: Such as Poliovirus which can survive the stomach's acidic environment.

Immunity Cells and Proteins Overview

  • Cell Types:

    • Agranulocytes: B cells, T cells, Monocytes

    • Granulocytes: Neutrophils, Eosinophils, Basophils

    • Phagocytes: Cells that consume foreign or damaged cells.

    • Natural Killer (NK) Cells: Part of innate immunity, targeting cancerous or virally infected cells.

  • Types of Proteins:

    • Antibodies: Secreted by B lymphocytes for adaptive immunity.

    • Complement System: Involved in innate immunity to facilitate pathogen destruction.

    • Cytokines: Proteins secreted by immune cells to regulate immune system activity.

Lymphatic and Immune System Interplay

  • Lymphoid organs provide housing for immune cells and trap pathogens for immune system action.

    • Dendritic cells activate T cells, and effective selection occurs in the thymus to ensure T cells are capable of functioning.

Cells of Innate Immunity

  • Phagocytosis: Involves the ingestion of particles by cells, including macrophages that can also secrete substances to destroy larger pathogens.

  • Neutrophils: Highly effective phagocytes that generate a respiratory burst upon activation.

  • Dendritic Cells: Play a significant role by presenting antigens to T cells for activation.

Antimicrobial Proteins

  • Complement System Overview: About 30 plasma proteins from the liver that can be activated via different pathways (classical, lectin, alternative) leading to pathogen lysis, inflammation enhancement, and opsonization.

  • Cytokines: Released from immune cells, involved in diverse responses including fever and recruitment of immune cells. Interleukins stimulate various immune functions, with some like interferons additionally having viral suppression activities.

Inflammatory Response

  • Definition: A series of events following tissue damage aiming to contain and repair the affected area.

  • Stages:

    • Release of inflammatory mediators causing changes in local tissue (redness, heat, swelling, pain).

    • Phagocyte arrival leading to the clean-up process and healing initiation.

Fever

  • Definition: An elevated body temperature indicating inflammatory processes, usually initiated by pyrogens that act on the hypothalamus.

  • Physiological change: Enhances phagocyte efficiency and is often accompanied by flu-like symptoms. Anti-pyretics may be used to manage high fevers.

Summary of Immune Responses

  • Primary and Secondary Responses: The primary response includes a lag phase and gradual antibody rise; the secondary response is faster and more robust due to memory B cells.

  • Vaccination: Exposes the individual to antigens to stimulate memory cell production, enhancing future responses.

  • Types of Vaccines: Include live-, inactivated-, subunit-, and mRNA vaccines with various applications in human health.

  • Challenges: High mutation rates in some pathogens complicate vaccine efficacy.

Disorders of the Immune System

  • Types:

    • Hypersensitivity Disorders: Such as allergies and anaphylaxis, where the immune response is excessive.

    • Immunodeficiency Disorders: Including primary genetic causes and secondary acquired conditions affecting immune strength.

    • Autoimmune Disorders: When the immune system mistakenly attacks the body’s own cells.

Hypersensitivity Disorders

  • Examples: Immediate hypersensitivity originating from allergens leading to responses like hives or anaphylaxis. Treatment often includes antihistamines and immunotherapy.

Immunodeficiency Disorders

  • Overview: Encompasses congenital conditions (like SCID) and acquired conditions (like AIDS), leading to increased susceptibility to infections.

Autoimmune Disorders

  • Pathologies where the immune system fails to recognize self from non-self, potentially leading to conditions like multiple sclerosis or Type 1 diabetes.

Conclusion of Immune Response Fundamentals

  • The intricate cooperation between the lymphatic and immune systems is pivotal for maintaining health, influencing responses to pathogens, and managing tissues throughout life. Understanding these mechanisms is essential for medical intervention and the treatment of diseases affecting these systems.