Chapter 23: The Lymphatic System & Immunity – Study Guide

The Lymphatic System

Overview and Functions

The lymphatic system is a network of vessels, tissues, and organs that helps maintain fluid balance, absorbs dietary lipids, and provides immune defense. It works closely with the cardiovascular and immune systems.

• Returns tissue fluid: Collects excess interstitial fluid and returns it to the bloodstream, maintaining blood volume.

• Transports dietary lipids: Absorbs fats and fat-soluble vitamins from the digestive tract via specialized lymphatic vessels called lacteals.

• Immune defense: Provides sites for immune cell activation and proliferation.

Primary and Secondary Lymphatic Organs

• Primary lymphatic organs: Sites where lymphocytes are produced and mature.

  • Red bone marrow: Produces all lymphocytes; B cells mature here.

  • Thymus: T cells mature here.

• Secondary lymphatic organs: Sites where immune responses are initiated.

  • Lymph nodes

  • Spleen

  • Tonsils

  • MALT (Mucosa-associated lymphoid tissue)

Lymph Flow and Lymphatic Capillaries

• Lymph flow: Always directed toward the heart, ensuring return of fluid to the venous system.

• Lymphatic capillaries:

  • Blind-ended (closed at one end)

  • Thinner walls than blood capillaries

  • Highly permeable to allow uptake of fluid, proteins, and cells

Major Lymphatic Structures

Lymphatic Ducts

Structure	Drainage Area	Drains Into
Thoracic duct	Left head/neck, left arm, left thorax, entire lower body	Left subclavian vein
Right lymphatic duct	Right head/neck, right arm, right thorax	Right subclavian vein

Lymphocytes

• B cells: Produce antibodies; mature in bone marrow.

• T cells: Mediate cellular immunity; mature in thymus.

  • Helper T cells: Activate other immune cells.

  • Cytotoxic T cells: Destroy infected or abnormal cells.

  • Regulatory T cells: Suppress immune responses to prevent autoimmunity.

  • Memory T cells: Provide long-term immunity.

• NK (Natural Killer) cells: Attack virus-infected and cancerous cells.

Lymphoid Tissues and Organs

Diffuse Lymphatic Tissue and Nodules

• Diffuse lymphatic tissue: Loosely arranged lymphocytes found in mucous membranes throughout the body.

• Lymphatic nodules: Densely packed clusters of lymphocytes in reticular connective tissue.

• MALT (Mucosa-associated lymphoid tissue): Lymphoid tissue associated with mucosal surfaces (e.g., digestive, respiratory tracts).

Tonsils and Peyer's Patches

• Tonsils: Five total—1 pharyngeal (adenoid), 2 palatine, 2 lingual; protect against inhaled or ingested pathogens.

• Peyer's patches: Aggregated lymphatic nodules in the wall of the small intestine, important for gut immunity.

Lymph Nodes

• Function: Filter lymph, trapping pathogens and debris.

• Structure: Many afferent (incoming) vessels, one efferent (outgoing) vessel to ensure slow filtration.

• Major node groups: Cervical (neck), axillary (armpit), inguinal (groin).

The Spleen

Structure and Function

Region	Function
Red pulp	Macrophages remove old or damaged red blood cells (RBCs)
White pulp	Contains lymphocytes; site of immune surveillance and response

• Primary functions:

  • Filters blood

  • Removes aged or damaged RBCs

  • Initiates immune responses to blood-borne antigens

Example: Lymphatic System in Action

• When bacteria enter a cut, lymphatic capillaries absorb the pathogens and transport them to nearby lymph nodes, where immune cells mount a defense.