Lymphatic System and Immunity: Structure and Function

Lymphatic System & Immunity

Overview

The lymphatic system is a crucial component of the body’s immune defense, working alongside the circulatory system to maintain fluid balance, transport dietary fats, and protect against disease. It consists of lymphatic vessels, lymphoid tissues, and organs.

• Structures: Lymphatic vessels, lymphoid tissues, and organs

• Functions:

  • Drain interstitial fluid and leaked plasma proteins back to blood

  • Transport dietary fats

  • Protect the body from disease

Review: Fluid Exchange and Bulk Flow

Bulk Flow in Capillaries

Bulk flow refers to the movement of fluid along hydrostatic and osmotic pressure gradients, regulating the volume of blood and interstitial fluid.

• Hydrostatic pressure: Pushes fluid out of capillaries

• Osmotic pressure: Draws fluid back into capillaries

• More fluid leaves blood capillaries than returns, with excess collected by lymphatic vessels

• Imbalance: Can lead to edema (swelling) or shock

Lymphatic Vessels

Structure and Function

• Principal lymphatic trunks unite into two main channels:

  • Right lymphatic duct: Drains right side of head, neck, chest, and arm

  • Thoracic duct: Drains all remaining regions of the body

• Both ducts drain into the subclavian veins

• Lymphatic capillaries: Blind-ended, collect excess interstitial fluid (now called lymph)

Lymphatic Circulation

Mechanisms of Lymph Flow

• Overlapping cells and valves prevent backflow of lymph

• Lymph flows toward the heart via:

  • Skeletal muscle contraction

  • Pressure changes from the respiratory pump

Lymphatic Organs

Primary Lymphatic Organs

• Thymus: Site where T lymphocytes mature and become immunocompetent; located between the sternum and heart; largest in childhood

• Red bone marrow: Site of lymphocyte production and B cell maturation

Secondary Lymphatic Organs

• Lymph nodes: Encapsulated structures along lymphatic vessels; filter substances and destroy pathogens via T cells, B cells, and macrophages

• Lymph nodules: Non-encapsulated; found in mucous membranes (e.g., tonsils, MALT)

• Spleen: Largest mass of lymphoid tissue; located inferior to the stomach in the left upper quadrant (LUQ)

Lymphatic Organs: Spleen

Structure and Function

• White pulp: Lymphatic tissue containing mostly T and B cells, and some macrophages; involved in immune responses

• Red pulp: Contains red blood cells (RBCs), platelets, white blood cells (WBCs), and mostly macrophages; functions in filtering blood, removing old RBCs, and storing platelets

Summary Table: Lymphatic Organs

Organ	Location	Main Function	Key Features
Thymus	Between sternum & heart	T cell maturation	Largest in childhood; primary lymphatic organ
Red Bone Marrow	Within bones	Lymphocyte production; B cell maturation	Primary lymphatic organ
Lymph Nodes	Along lymphatic vessels	Filter lymph; immune surveillance	Encapsulated; contains T, B cells, macrophages
Lymph Nodules	Mucous membranes (e.g., tonsils, MALT)	Immune response at mucosal surfaces	Non-encapsulated
Spleen	Inferior to stomach, LUQ	Filters blood; immune response	White pulp (lymphatic tissue), red pulp (RBCs, macrophages)

Key Terms

• Lymph: Fluid collected from interstitial spaces, returned to blood via lymphatic vessels

• Edema: Swelling caused by excess interstitial fluid

• Immunocompetent: Ability of lymphocytes to recognize and respond to antigens

• MALT: Mucosa-associated lymphoid tissue; protects mucosal surfaces

Example

If lymphatic vessels are blocked or damaged, interstitial fluid cannot be efficiently returned to the bloodstream, resulting in edema. This highlights the importance of the lymphatic system in maintaining fluid balance and preventing tissue swelling.