Chapter 20: The Lymphatic System and Body Defenses

Part I: The Lymphatic System

Overview of the Lymphatic System

The lymphatic system is a crucial component of the body's circulatory and immune systems, consisting of lymphatic vessels and lymphoid tissues and organs. It plays essential roles in fluid balance and defense against disease.

• Lymphatic vessels: Network of tubes that transport lymph.

• Lymphoid tissues and organs: Include lymph nodes, spleen, thymus, tonsils, and Peyer's patches.

• Functions:

  • Transports escaped fluids (interstitial fluid) back to the blood.

  • Contributes to body defense and resistance to disease.

Lymphatic Vessels

Lymphatic vessels collect and return excess tissue fluid and plasma proteins to the bloodstream, preventing edema and maintaining fluid balance.

• Lymph: Fluid carried by lymphatic vessels; derived from interstitial fluid.

• If not removed, excess fluid causes edema (swelling).

• Lymphatic vessels form a one-way system toward the heart.

Lymph Capillaries

• Weave between tissue cells and blood capillaries.

• Walls overlap to form flaplike minivalves that allow fluid entry.

• Higher internal pressure closes minivalves, forcing fluid along the vessel.

Lymphatic Collecting Vessels

• Collect lymph from capillaries and transport it to and from lymph nodes.

• Return fluid to circulatory veins near the heart.

• Right lymphatic duct: Drains lymph from right arm, right side of head and thorax.

• Thoracic duct: Drains lymph from the rest of the body.

Structural Features

• Similar to veins: thin-walled, larger vessels have valves, low-pressure, pumpless system.

• Lymph transport aided by:

  • Milking action of skeletal muscles

  • Pressure changes during breathing

  • Smooth muscle contraction in vessel walls

Lymph Nodes

Lymph nodes are small, bean-shaped structures that filter lymph before it returns to the blood, removing harmful materials and supporting immune function.

• Filtered materials: Bacteria, viruses, cancer cells, cell debris.

• Defense cells:

  • Macrophages: Engulf and destroy foreign substances.

  • Lymphocytes: Respond to foreign substances in the lymphatic system.

Structure of a Lymph Node

• Capsule, cortex, medulla, follicles, germinal centers, afferent/efferent vessels.

Other Lymphoid Organs

Several organs contribute to lymphatic function and immune defense.

• Spleen: Filters blood, destroys worn-out blood cells, acts as a blood reservoir.

• Thymus: Produces thymosin to program lymphocytes, most active during youth.

• Tonsils: Trap and remove bacteria and other foreign materials.

• Peyer's Patches: Capture and destroy bacteria in the intestine.

Part II: Body Defenses

Overview of Body Defenses

The body is constantly exposed to pathogens such as bacteria, fungi, and viruses. Two main defense systems protect against these threats: innate (nonspecific) and adaptive (specific) immunity.

• Innate (nonspecific) defense system: Provides immediate, general protection against a variety of invaders.

• Adaptive (specific) defense system: Provides targeted defense against specific pathogens.

• Immunity: Specific resistance to disease.

Innate (Nonspecific) Defenses

• First line of defense: Skin, mucous membranes, secretions.

• Second line of defense: Phagocytic cells, natural killer cells, antimicrobial proteins, inflammatory response.

• Responds immediately to protect the body from foreign materials.

Adaptive (Specific) Defenses

• Third line of defense: Lymphocytes, antibodies, antigen-presenting cells.

• Specific defense is required for each type of invader.

Table: Overview of the Immune System

Innate (Nonspecific) Defense Mechanisms	Adaptive (Specific) Defense Mechanisms
First line of defense: Skin Mucous membranes Secretions of skin and mucous membranes	Third line of defense: Lymphocytes Antibodies Macrophages and other antigen-presenting cells
Second line of defense: Phagocytic cells Natural killer cells Antimicrobial proteins The inflammatory response

Inflammatory Response

The inflammatory response is triggered when body tissues are injured, serving to prevent the spread of damaging agents, dispose of cell debris and pathogens, and set the stage for tissue repair.

• Indicators of acute inflammation:

  1. Redness

  2. Heat

  3. Swelling

  4. Pain

• Process:

  1. Neutrophils migrate to the area of inflammation by rolling along the vessel wall.

  2. Neutrophils squeeze through capillary walls by diapedesis.

  3. Neutrophils gather at the injury site by positive chemotaxis and consume foreign material.

Adaptive Body Defenses: Types of Immunity

• Cellular immunity (cell-mediated): Targets virus-infected cells, cancer cells, and cells of foreign grafts.

• Humoral immunity (antibody-mediated): Provided by antibodies present in body fluids.

Active and Passive Humoral Immunity

• Active humoral immunity:

  • Naturally acquired during bacterial and viral infections.

  • Artificially acquired from vaccines.

• Passive humoral immunity:

  • Occurs when antibodies are obtained from another source.

  • Naturally acquired from mother to fetus via placenta or to infant in milk.

  • Artificially acquired from immune serum or gamma globulin.

Table: Types of Acquired Immunity

	Naturally Acquired	Artificially Acquired
Active	Infection; contact with pathogen	Vaccine; dead or attenuated pathogens
Passive	Antibodies pass from mother to fetus via placenta or to infant in her milk	Injection of immune serum (gamma globulin)

Disorders of the Immune System

• Autoimmune diseases: Occur when the body's self-tolerance breaks down, leading to immune attack on its own tissues (e.g., rheumatoid arthritis, multiple sclerosis, Type I diabetes).

• Allergies: Abnormal, vigorous immune responses to harmless antigens, resulting in tissue destruction (e.g., hay fever, hives, anaphylaxis).

• Immunodeficiencies: Result from abnormalities in immune elements, leading to abnormal production or function of immune cells (e.g., AIDS).