Chapter 20: The Lymphatic System and Immunity – Study Notes

20.1 Structure and Function of the Lymphatic System

Overview of the Lymphatic System

The lymphatic system is a network of organs and tissues that collaborates with the immune system to defend the body against pathogens and maintain fluid balance. It consists of two main components:

• Lymphatic Vessels: A system of blind-ended vessels that transport lymph.

• Lymphatic Tissue and Lymphoid Organs: Includes clusters of lymphoid follicles (e.g., tonsils), lymph nodes, spleen, and thymus.

Functions of the Lymphatic System

• Regulation of Interstitial Fluid Volume: Blood capillaries lose about 2–4 liters of fluid per day to the interstitial space. Lymphatic vessels collect this fluid (now called lymph) and return it to the cardiovascular system, maintaining fluid homeostasis.

• Absorption of Dietary Fats: Specialized lymphatic vessels in the small intestine absorb breakdown products of dietary fats and deliver them to the bloodstream.

• Immune Functions: Lymphoid organs filter pathogens from lymph and blood and house leukocytes (white blood cells) involved in immune responses.

20.1 Lymphatic Vessels and Lymph Circulation

Lymph Circulation Pathways

Lymph-collecting vessels merge to form nine major lymph trunks, each draining specific body regions:

• Lumbar Trunks: Lower limbs and pelvic area

• Intestinal Trunk: Fat-containing lymph from the small intestine

• Jugular Trunks: Head and neck

• Bronchomediastinal Trunks: Thoracic cavity

• Subclavian Trunks: Upper limbs

The cisterna chyli receives lymph from the intestinal and lumbar trunks and drains into the thoracic duct, which empties into the junction of the left internal jugular and left subclavian veins. The right lymphatic duct drains the right upper side of the body into the right internal jugular and right subclavian veins.

Mechanisms of Lymph Flow

• Lymphatic vessels form a low-pressure circuit (no central pump).

• Lymphatic valves prevent backflow of lymph.

• Vessels are often located between muscles; muscle contractions help propel lymph toward the heart.

• Contractions of smooth muscle in vessel walls also aid lymph movement.

Lymphedema

• Edema is the accumulation of excess interstitial fluid, which can be mild or severe.

• Lymphedema is a severe, often disfiguring swelling, usually due to surgical removal or blockage of lymphatic vessels (e.g., after cancer surgery or due to parasitic infection).

• Fluid accumulates in the affected area, causing enlargement.

Specialized Lymphatic Structures

• Lacteals: Specialized lymphatic capillaries in the small intestine that absorb dietary fats.

• Lymphatic Capillaries: Blind-ended, weblike networks around blood capillaries; endothelial cells form flaps that open and close in response to pressure changes, allowing fluid, macrophages, bacteria, and cancer cells to enter.

20.1 Lymphoid Tissues and Organs

Reticular Tissue (Lymphoid Tissue)

The predominant tissue type in the lymphatic system is reticular tissue, which contains:

• Reticular fibers: Form nets to trap pathogens and specialized cells.

• Lymphocytes: B and T cells (adaptive immunity).

• Phagocytes: Macrophages (mature monocytes) and dendritic cells (antigen-presenting cells).

• Reticular cells: Produce reticular fibers (specialized collagen).

Mucosa-Associated Lymphatic Tissue (MALT)

• Loosely organized clusters of lymphoid tissue protecting mucous membranes exposed to pathogens.

• Located throughout the gastrointestinal tract, respiratory passages, and genitourinary tract.

• Most MALT consists of clusters of B and T cells without a connective tissue capsule; some are encapsulated.

• Specialized MALT includes lymphoid follicles (nodules) with germinal centers (active B cell division).

Examples of MALT:

• Tonsils: Pharyngeal (adenoid), palatine, and lingual tonsils; tonsillar crypts trap pathogens and may cause tonsillitis.

• Peyer's patches: Aggregated lymphoid nodules in the ileum of the small intestine, defending against bacteria from the large intestine.

• Appendix: Defends against bacteria in the large intestine; susceptible to appendicitis if blocked.

Lymph Nodes

• Small, bean-shaped clusters of lymphatic tissue along lymphatic vessels.

• Found in clusters: axillary (armpit), cervical (neck), inguinal (groin), and mesenteric (abdominal) lymph nodes.

• Each node has an external connective tissue capsule and internal reticular fibers filled with immune cells.

• Structure:

  • Outer cortex: B cell-rich lymphoid follicles separated by trabeculae.

  • Medulla: Medullary cords with macrophages and mature B cells; T cells at the cortex-medulla junction.

• Lymph enters via afferent lymphatic vessels, passes through the subcapsular sinus and cortical sinuses (where immune responses may be initiated), then through medullary sinuses, and exits via efferent lymphatic vessels at the hilum.

• Lymph nodes filter about 90% of pathogens from lymph.

Example Table: Main Lymphatic Trunks and Their Drainage Regions

Lymphatic Trunk	Drains Region
Lumbar Trunks	Lower limbs, pelvic area
Intestinal Trunk	Small intestine (fat-containing lymph)
Jugular Trunks	Head and neck
Bronchomediastinal Trunks	Thoracic cavity
Subclavian Trunks	Upper limbs

Example:

A patient who has undergone surgical removal of axillary lymph nodes for breast cancer treatment may develop lymphedema in the affected arm due to impaired lymph drainage.

Additional info: The lymphatic system is essential for both immune defense and fluid balance. Disruption of lymphatic flow can lead to clinical conditions such as lymphedema, and the organization of lymphoid tissues is critical for effective immune surveillance and response.