Chapter 20 - BLG 102 - The Lymphatic System and Lymphoid Organs and Tissues

The lymphatic system has the primary function of returning fluids that leak from blood vessels back to the blood.
It is composed of three main components:
1. Network of lymphatic vessels (lymphatics)
2. Lymph: The fluid contained within these vessels
3. Lymph nodes: Structures that cleanse lymph
Lymphoid organs and tissues provide the structural basis of the immune system by housing phagocytic cells and lymphocytes.
- Key structures include:
- Spleen
- Thymus
- Tonsils
- Lymph nodes
- Other lymphoid tissues

The lymphatic system returns interstitial fluid and leaked plasma proteins back to the blood via:
- Lymphatic vessels (lymphatics), which form an elaborate network of drainage vessels that circulates approximately 3 liters of interstitial fluid per day.
Upon entering lymphatic vessels, interstitial fluid is referred to as lymph.

Lymphatic vessels create a one-way system ensuring that lymph flows only toward the heart.
Types of lymphatic vessels:
- Lymphatic capillaries: Blind-ended vessels that intersperse between tissue cells and blood capillaries.
- Characteristics:
  - Absent in bones, teeth, and bone marrow
  - More permeable than blood capillaries, capable of absorbing larger molecules and particles, such as proteins, cell debris, pathogens, and cancer cells.
- They provide a potential route for pathogens or cancer cells to disperse throughout the body.
- Larger lymphatic vessels: Formed when lymph capillaries drain into increasingly larger vessels known as collecting lymphatic vessels, which consist of:
- Collecting vessels
- Trunks
- Ducts
  - These structures have features similar to veins but with thinner walls and more internal valves, and they often anastomose more frequently.

Increased permeability of lymphatic capillaries results from specialized structures:
- Endothelial cells that overlap to form one-way minivalves.
- Minivalves are held open by collagen filaments; increases in extracellular fluid (ECF) volume cause these minivalves to open wider, while decreases in ECF volume cause them to close.
- Lacteals: Specialized lymph capillaries present in the intestinal mucosa that absorb digested fats and deliver a fatty lymph, known as chyle, into the bloodstream.

Lymph delivery: From lymphatic trunks into two major ducts.
- Right lymphatic duct: Drains lymph from the right upper arm and right side of the head and thorax, emptying into the blood at the junction of the right internal jugular and subclavian veins.
- Thoracic duct: Drains lymph from the rest of the body, starting as an enlarged sac called cisterna chyli and emptying into the blood at the junction of the left internal jugular and subclavian veins.

The lymphatic system operates as a low-pressure system, similar to the venous system.
Lymph transport is facilitated by:
- Milking action of skeletal muscles
- Pressure changes during breathing
- Valves that prevent backflow
- Pulsations of adjacent arteries
- Contraction of smooth muscle within lymphatic walls
Enhanced physical activity increases lymph flow, while immobilization helps to retain necessary inflammatory materials for quick healing.

Lymphoid cells consist of two types:
1. Immune system cells located within lymphoid tissue
2. Supporting cells that form structures in lymphoid tissue

Lymphocytes: Integral cells of the adaptive immune system, maturing into two main types:
- T cells (T lymphocytes): Manage immune responses and attack infected cells.
- B cells (B lymphocytes): Produce plasma cells secreting antibodies that mark antigens for destruction via phagocytosis or other means.
Lymphocytes protect against antigens, defined as any substances perceived as foreign, including bacteria, toxins, viruses, mismatched RBCs, and cancer cells.

Macrophages: Phagocytize foreign substances and assist in activating T cells.
Dendritic cells: Capture antigens and present them to lymph nodes, contributing to T cell activation.

Reticular cells: Produce reticular fibers known as stroma in lymphoid organs.
Stroma: A network-like structure that acts as a scaffold for immune cells.

Main functions of lymphoid tissue:
1. Housing and providing sites for the proliferation of lymphocytes.
2. Offering surveillance vantage points for lymphocytes and macrophages to filter through lymph.
Comprised mainly of reticular connective tissue, which allows macrophages to reside on reticular fibers, providing space for lymphocytes upon their return from body patrols.

Primary lymphoid organs:
- Thymus
- Red bone marrow
Secondary lymphoid organs:
- Lymph nodes
- Tonsils
- Spleen
- Peyer’s patches (aggregated lymphoid nodules in the small intestine)
- Appendix

Lymph nodes are the principal secondary lymphoid organs, with hundreds located throughout the body.
They are mostly embedded deep in connective tissue in clusters alongside lymphatic vessels but can also be found nearer the body surface in areas such as the inguinal, axillary, and cervical regions, where collecting vessels converge into trunks.

Two primary functions of lymph nodes include:
1. Cleansing the lymph: Acting as filters where macrophages remove and destroy microorganisms and debris, preventing unwanted substances from entering the bloodstream.
2. Immune system activation: Providing a venue for lymphocytes to become activated and mount a defensive response against antigens.