Lymphatic System and Immunity: Structure and Function

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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, absorb 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: Bulk Flow and Fluid Balance

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.

More fluid (~3L/day) leaves blood capillaries than returns, which could lead to edema or shock if not properly drained.

Lymphatic Vessels

Structure and Function

Lymphatic Capillaries: Blind-ended vessels that collect excess interstitial fluid (now called lymph).
Lymph drains into larger lymphatic vessels and is eventually returned to the cardiovascular system.

Main Lymphatic Ducts

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 Circulation

Mechanisms of Lymph Flow

Overlapping cells and valves prevent backflow of lymph.
Lymph flows to 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 located along lymphatic vessels; filter substances and destroy pathogens via T cells, B cells, and macrophages.
Lymph nodules: Non-encapsulated lymphatic tissue found in mucous membranes (e.g., MALT—mucosa-associated lymphoid tissue).
Spleen: Largest mass of lymphoid tissue in the body, located inferior to the stomach in the left upper quadrant (LUQ).

Lymph Nodes vs. Lymph Nodules

Feature	Lymph Nodes	Lymph Nodules
Capsule	Present	Absent
Location	Along lymphatic vessels	Mucous membranes (MALT)
Function	Filter lymph, destroy pathogens	Immune surveillance in mucosa

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 the removal of old RBCs and platelets, and storage of platelets.

Summary Table: Spleen Tissue Types

Tissue Type	Main Components	Function
White pulp	T & B lymphocytes, macrophages	Immune response to blood-borne antigens
Red pulp	RBCs, platelets, WBCs, macrophages	Phagocytosis of old RBCs, platelet storage

Key Terms

Lymph: Interstitial fluid collected by lymphatic capillaries and returned to the bloodstream.
Edema: Swelling caused by excess fluid in tissues, often due to impaired lymphatic drainage.
Immunocompetent: The ability of lymphocytes to recognize and respond to antigens.
MALT: Mucosa-associated lymphoid tissue, important for immune defense in mucous membranes.

Additional info: The lymphatic system is essential for both fluid homeostasis and immune surveillance, making it a key topic in Anatomy & Physiology.