Chapter 20: The Lymphatic System and Lymphoid Organs and Tissues

Chapter 20: The Lymphatic System and Lymphoid Organs and Tissues

Overview of the Lymphatic System

The lymphatic system is essential for returning fluids leaked from blood vessels back to the bloodstream and for supporting immune function. It consists of three main components:

• Network of lymphatic vessels (lymphatics): These vessels transport lymph throughout the body.

• Lymph: The fluid found within lymphatic vessels, derived from interstitial fluid.

• Lymphoid organs and tissues: Structures that cleanse lymph and provide the structural basis for the immune system by housing phagocytic cells and lymphocytes. Examples include the thymus, tonsils, lymph nodes, and other lymphoid tissues.

The lymphatic system returns interstitial fluid and leaked plasma proteins to the blood via an elaborate network of drainage vessels. Once interstitial fluid enters lymphatics, it is called lymph.

Lymphatic Vessels

Lymphatic vessels form a one-way system, ensuring lymph flows only toward the heart. The smallest vessels, lymphatic capillaries, are blind-ended and weave between tissue cells and blood capillaries. They are more permeable than blood capillaries and can take up larger molecules and particles, such as proteins, cell debris, pathogens, and cancer cells.

• Increased permeability is due to two specialized structures:

  • Minivalves: Overlapping endothelial cells form one-way valves.

  • Lacteals: Specialized lymph capillaries in the intestinal mucosa that absorb digested fats and deliver fatty lymph (chyle) to the blood.

• Lymph capillaries drain into larger vessels called collecting lymphatic vessels, which consist of collecting vessels, trunks, and ducts. These are similar to veins but are thinner and have more frequent anastomoses.

Lymphatic Trunks and Ducts

Lymphatic trunks are formed by the union of the largest collecting vessels and drain large areas of the body. They are named for the regions they drain:

• Paired lumbar trunks

• Paired bronchomediastinal trunks

• Paired subclavian trunks

• Paired jugular trunks

• Single intestinal trunk

Lymph is delivered from trunks into one of two large lymphatic ducts:

• Right lymphatic duct: Drains the right upper arm and right side of the head and thorax.

• Thoracic duct: Drains the rest of the body.

Each duct empties lymph into venous circulation at the junction of the internal jugular and subclavian veins on its own side of the body.

Lymphatic Vessel Disorders

• Lymphangitis: Condition where lymphatic vessels appear as painful red lines under the skin, caused by inflammation of larger lymphatic vessels that contain vasa vasorum. The vasa vasorum becomes congested with blood.

• Lymphedema: Severe localized edema caused by anything that prevents normal return of lymph to blood, such as tumors or removal of lymphatics.

Lymphoid Cells

Lymphoid cells include:

• Immune system cells:

  • Lymphocytes: Cells of the adaptive immune system. Two types:

    • T cells (T lymphocytes): Manage immune response and attack infected cells.

    • B cells (B lymphocytes): Produce plasma cells, which secrete antibodies. Antibodies mark antigens for destruction by phagocytosis or other means.

  • Macrophages: Phagocytize foreign substances and help activate T cells.

  • Dendritic cells: Capture antigens and deliver them to lymph nodes; also help activate T cells.

• Supporting lymphoid cells: Reticular cells produce reticular fibers called stroma, which act as scaffolding for immune cells.

Lymphoid Tissue

Lymphoid tissue provides sites for lymphocyte proliferation and offers surveillance vantage points for lymphocytes and macrophages as they filter through lymph. It is mainly composed of reticular connective tissue.

• Diffuse lymphoid tissue: Loose arrangement of lymphoid cells and some reticular fibers, found in virtually every body organ.

• Lymphoid follicles (nodules): Solid, spherical bodies consisting of tightly packed lymphoid cells and reticular fibers. Contain germinal centers of proliferating B cells and may form part of larger lymphoid organs.

Lymphoid Organs

Lymphoid organs are classified as primary or secondary:

• Primary lymphoid organs: Sites where T and B cells mature. Includes red bone marrow (B cells) and thymus (T cells).

• Secondary lymphoid organs: Sites where mature lymphocytes first encounter antigens and become activated. Includes lymph nodes, spleen, MALT (mucosa-associated lymphoid tissue), and diffuse lymphoid tissues.

Lymph Nodes

Lymph nodes are the principal secondary lymphoid organs of the body. They are located in deep connective tissue and have two main functions:

• Cleansing of the lymph: Act as lymph filters; macrophages remove and destroy microorganisms and debris.

• Immune system activation: Provide a place for lymphocytes to become activated and mount an attack against antigens.

Structure:

• Surrounded by an external fibrous capsule.

• Capsule fibers extend inward as trabeculae that divide the node into compartments.

• Two histologically distinct regions:

  • Cortex: Superficial area contains follicles with germinal centers (dividing B cells); deep cortex contains T cells in transit and abundant dendritic cells.

  • Medulla: Medullary cords extend inward from cortex and contain B cells, T cells, and plasma cells. Medullary sinuses contain large lymphatic capillaries spanned by reticular fibers; macrophages reside on fibers.

Lymph enters the convex side of the node via afferent lymphatic vessels, travels through the subcapsular sinus and smaller sinuses, enters medullary sinuses, and finally exits the concave side at the hilum via efferent lymphatic vessels.

• Buboes: Inflamed, swollen, tender lymph nodes overwhelmed by what they are destroying; often referred to as swollen glands. Bubonic plague was named after this clinical feature.

• Imbalances: Lymph nodes can become sites for cancer cells if trapped.

Spleen

The spleen is a blood-rich organ located in the left side of the abdominal cavity, just below the stomach. It is the largest lymphoid organ and is served by the splenic artery and vein, which enter and exit at the hilum.

• Functions:

  • Site of lymphocyte proliferation and immune surveillance/response

  • Cleanses blood of aged red blood cells (RBCs) and platelets; macrophages remove debris

  • Stores breakdown products of RBCs (e.g., iron) for later use

  • Stores blood platelets and monocytes for release into blood when needed

  • Site of fetal erythrocyte production

• Splenectomy: Surgical removal of a ruptured spleen. If removed, liver and bone marrow take over most functions. In children under 12, the spleen can regenerate if a small part is left.

The spleen has two components:

• White pulp: Immune function

• Red pulp: Where old RBCs are broken down

Mucosa-Associated Lymphoid Tissue (MALT)

MALT consists of lymphoid tissues in mucous membranes throughout the body, protecting from pathogens trying to enter. Largest collections are found in:

• Tonsils: Simplest lymph organ; includes palatine, lingual, pharyngeal, and tubal tonsils

• Peyer's patches: Clusters of lymphatic follicles in the small intestine

• Appendix: Offshoot of the large intestine; generates memory lymphocytes

Thymus

The thymus is a bilobed lymphoid organ found in the inferior neck. It functions as the site where T cells mature. The thymus is broken into lobules containing an outer cortex (rapidly dividing lymphocytes and scattered macrophages) and an inner medulla (fewer lymphocytes and thymic corpuscles).

• Thymic corpuscles: Where regulatory T cells develop; these cells help prevent autoimmunity.

• Type: The thymus lacks follicles and B cells.

Summary Table: Lymphatic System Components

Component	Main Function	Key Features
Lymphatic Vessels	Transport lymph	One-way flow, minivalves, lacteals
Lymph Nodes	Filter lymph, immune activation	Cortex (B cells), medulla (B/T/plasma cells), afferent/efferent vessels
Spleen	Filter blood, immune response	White pulp (immune), red pulp (RBC breakdown)
MALT	Protect mucosal surfaces	Tonsils, Peyer's patches, appendix
Thymus	T cell maturation	No follicles, lacks B cells, thymic corpuscles

Key Terms and Definitions

• Lymph: Fluid in lymphatic vessels, derived from interstitial fluid.

• Lymphocytes: White blood cells involved in adaptive immunity (T cells and B cells).

• Antigen: Any substance the body perceives as foreign (e.g., bacteria, viruses, toxins).

• Macrophage: Phagocytic cell that engulfs pathogens and debris.

• Dendritic cell: Antigen-presenting cell that activates T cells.

• Stroma: Reticular fiber network supporting immune cells in lymphoid organs.

• Minivalve: One-way valve formed by overlapping endothelial cells in lymphatic capillaries.

• Lacteal: Specialized lymphatic capillary in the intestinal mucosa for fat absorption.

• Buboes: Swollen lymph nodes due to infection.

• Splenectomy: Surgical removal of the spleen.

• MALT: Mucosa-associated lymphoid tissue.

• Thymic corpuscle: Structure in thymus where regulatory T cells develop.

Additional info:

• Regulatory T cells are a subset of T cells that help prevent autoimmunity by suppressing immune responses against self-antigens.

• Chyle is the milky fluid consisting of lymph and emulsified fats absorbed from the intestines.

• Imbalances in the lymphatic system can lead to conditions such as lymphedema or serve as sites for cancer metastasis.