Human Anatomy and Physiology: The Lymphatic System and Immunity

Human Anatomy and Physiology: Lymphatic System and Immunity

Learning Objectives

  • Describe the functions of the lymphatic system and lymphatic structures.

  • Describe the location, structure, and functions of the spleen, thymus, and lymph nodes.

  • Discuss the formation of lymph from tissue fluid and lymph flow through the major lymph vessels.

  • Briefly describe the structures and functions of the immune system, distinguishing between antigens and antibodies.

  • Describe the body’s three lines of defense against pathogens, including the difference between nonspecific (innate) and specific (adaptive) immunity.

  • Explain the function of specific T and B cells in the development of the adaptive immune response.

  • Discuss types of immunity: active and passive, natural and artificial, providing examples of each.

Lymphatics and Lymphoid Organs

Disease Resistance
Nonspecific Resistance (Innate Immunity)

  • Present at birth.

  • Includes defense mechanisms that provide general protection against a wide range of pathogens.

Immunity (Adaptive Immunity)

  • Involves activation of specific lymphocytes that combat particular pathogens or foreign substances.

  • The system that carries out immune responses is the lymphatic system.

Components of the Lymphatic System

  • The lymphatic system comprises various structures and organs that contain lymphatic tissue, bone marrow, and lymph, which flows within lymphatic vessels.

Structure and Functions of the Lymphatic System

The lymphatic system performs the following functions:

  1. Drain interstitial fluid

  2. Transport dietary fats

  3. Carry out immune responses

  • Lymphatic vessels begin as lymphatic capillaries, which are closed on one end.

  • Location: Lymphatic capillaries are present between cells of many tissues.

  • Lymphatic capillaries merge to form lymphatic vessels, characterized by thin walls and many valves.

Lymphatic Capillaries

  • Lymphatic capillaries interact closely with blood capillaries, absorbing interstitial fluid.

  • They are comprised of an endothelium and are anchored by filaments to surrounding tissues to prevent collapse under pressure.

Lymph Trunks and Ducts

  • After flowing through the lymphatic vessels, lymph passes through lymph nodes and into lymph trunks.

  • Two main ducts:

    • Thoracic Duct: Drains lymph from the lower limbs, abdomen, left side of the chest, left arm, and left head/neck.

    • Right Lymphatic Duct: Drains the right side of the upper body.

Routes of Drainage

  • Thoracic Duct: Drains lymph from areas including the left internal jugular vein.

  • Right Lymphatic Duct: Drains lymph from the right internal jugular vein and subclavian vein, eventually merging with superior vena cava.

Formation and Flow of Lymph

  • Pathway: Interstitial fluid → lymphatic capillaries → lymph vessels → lymph trunks → lymph ducts → subclavian veins.

Lymphatic Organs and Tissues

Primary Lymphatic Organs

  • Organs where immune cells become immunocompetent:

    • Red Bone Marrow

    • Thymus

Secondary Lymphatic Organs and Tissues

  • Include:

    • Lymph Nodes

    • Spleen

    • Lymphatic Nodules

    • Tonsils

    • Peyer’s Patches (in small intestine)

    • Appendix

Lymph Nodes

  • Principal secondary lymphoid organs found throughout the body with two main functions:

    1. Cleansing the lymph: Act as filters to remove microorganisms via phagocytosis.

    2. Immune system activation: Sites for lymphocytes to become activated against antigens.

Lymph Node Structure

  • Contains various cells:

    • T cells, B cells, dendritic cells, macrophages.

  • Organized into regions: outer cortex (with germinal centers) and inner medulla.

  • Lymph flows through afferent vessels into subcapsular sinus, trabecular sinus, and out through efferent vessels.

Spleen

  • A blood-rich organ about the size of a fist located on the left side of the abdominal cavity, below the stomach. Largest lymphoid organ.

  • Functions:

    • Site of lymphocyte proliferation and immune response.

    • Cleanses blood of aged cells and platelets via macrophages.

  • Vulnerable to rupture due to a thin capsule; can result in blood spilling into the peritoneal cavity.

Lymphatic Nodules

  • Masses of lymphatic tissue not surrounded by a capsule.

  • Found scattered in mucous membranes across the gastrointestinal, urinary, reproductive tracts, and respiratory airways.

  • Referred to as mucosa-associated lymphatic tissue (MALT).

  • Includes tonsils (pharyngeal), Peyer’s patches, and appendix.

Immunity

Innate Immunity

  • Refers to a variety of body responses to protect against pathogens present at birth.

  • Two Lines of Defense:

    1. Skin and Mucous Membranes: First line of defense.

    2. Internal Defenses: Second line of defense including phagocytes, natural killer cells, fever, and inflammation.

Skin and Mucous Membranes
Mechanical Defenses

  • Include: skin, mucous membranes, tears, saliva, cilia, urination, vomiting, and defecating.

Chemical Defenses

  • Include: sebum, lysozyme, gastric juice.

Internal Defenses

  • Includes:

    • Antimicrobial proteins.

    • Phagocytes: Cellular ingestion of pathogens.

    • Natural killer cells.

    • Inflammatory response.

    • Fever.

Phagocytosis Steps

  1. Chemotaxis: Movement of phagocytes toward an inflammatory site.

  2. Adherence: Phagocyte binds to microbe.

  3. Ingestion: Microbe is engulfed into a phagosome.

  4. Digestion: Phagosome merges with lysosome to digest the pathogen.

  5. Killing: Formation of residual body with indigestible material.

Adaptive Immunity

  • The body’s ability to defend against specific invading agents, including both specificity and memory.

  • Antigens: Substances recognized as foreign that provoke immune responses.

    • Reactivity: Ability of an antigen to bind to immune molecules.

    • Immunogenicity: Ability of an antigen to trigger an immune response.

    • Epitopes: Small parts of large antigen molecules that trigger immune responses.

Types of Adaptive Immunity

  1. Cell-Mediated Immunity: T-lymphocyte activation.

  2. Antibody-Mediated Immunity: B-lymphocyte activation.

T and B Cell Functionality

  • T Cells (mature in thymus) include:

    • Helper T Cells (CD4): Activate other immune cells (e.g., CD8 and B cells).

    • Cytotoxic T Cells (CD8): Kill infected or abnormal cells.

    • Memory T Cells: Maintain immune memory.

  • B Cells (mature in red bone marrow):

    • Differentiate into plasma cells that produce antibodies and memory B cells.

Antibodies

  • Produced by plasma cells to identify and neutralize antigens.

  • Types of Antibodies (Immunoglobulins):

    • IgG: Most common; protects against bacteria and viruses; crosses placental barrier.

    • IgM: First to respond during primary immune response, present as pentamers.

    • IgA: Found in secretions; localized protection in mucous membranes.

    • IgE: Involved in allergic reactions; protects against parasites.

    • IgD: Activates B cells; present on their surface.

Classes of Immunoglobulins

Class

Characteristics and Functions

IgG

Most abundant; 80% of all antibodies; important for protection and maternal fetal transfer.

IgA

Present in secretions; provides mucosal protection.

IgM

First antibody class synthesized in response to antigen; critical for agglutination.

IgE

Involved in hypersensitivity; provides protection against parasites.

IgD

Functions in B cell activation; present in small amounts.

Adaptive Immunity Classification

  • Two avenues of acquisition:

  1. Natural: Via natural exposure (infection) or maternal antibody transfer (breastfeeding).

  2. Artificial: Through vaccines or immunoglobulin injections.

  • Types:

    • Active: Person's immune system generates own antibodies (long-term immunity).

    • Passive: Person receives antibodies from an external source (short-term immunity).

Acquired Natural Immunity

  • Natural Active Immunity: Direct exposure leads to a primary immune response.

  • Natural Passive Immunity: Maternal antibodies provide temporary immunity to infants (6-8 months).

Acquired Artificial Immunity

  • Artificial Active Immunity: Introduction of vaccines stimulates antibody production.

  • Artificial Passive Immunity: Administration of ready-made antibodies (e.g., gamma globulin).

Matching Exercises

Lymphoid Organ Matching

  1. Has more afferent than efferent vessels - B. Lymph nodes

  2. Cleanses the blood - C. Spleen

  3. Called adenoids when enlarged - A. Tonsils

  4. Filters lymph - B. Lymph nodes

  5. Key immune system regulator - D. Thymus

  6. Where T-lymphocytes mature - D. Thymus

Immunity Matching

  1. Present in body secretions - C. IgA

  2. Antibody-mediated immunity - A. B-cells

  3. Artificial active immunity - E. Vaccine

  4. Artificial passive immunity - F. Gamma globulin shot

  5. Cell-mediated immunity - B. T-cells

  6. Crosses the placenta - D. IgG

Questions

  1. Children born without a thymus must be kept in a germ-free environment for survival due to the lack of T-cell maturation and immune response capacity.

  2. Lymph traveling from the left arm enters venous circulation via the thoracic duct.

  3. Obstruction of the lymphatics can lead to lymphedema, increasing fluid in tissues.

  4. Immunity resulting from antibodies that pass through the placenta is called passive immunity.

  5. Cells that help regulate the immune response are helper T cells.

  6. Adaptive defenses depend on the activities of T and B lymphocytes.

Order of Lymph Flow

  1. Interstitial fluid

  2. Lymphatic capillary

  3. Afferent lymphatic vessel

  4. Lymphatic node

  5. Efferent lymphatic vessel

  6. Lymphatic trunk

  7. Lymphatic duct

  8. Subclavian vein