Lymphatic and Immune Systems

Overview and Functions

The lymphatic and immune systems are essential for maintaining fluid balance, absorbing dietary lipids, and defending the body against pathogens. The lymphatic system consists of a network of vessels, organs, and tissues that filter interstitial fluid and facilitate immune responses.

• Immunity: Filters interstitial fluid from capillary beds; immune cells respond to foreign cells or chemicals.

• Lipid Absorption: Transports fats from the intestine to the bloodstream.

• Fluid Recovery: Absorbs plasma proteins and fluid from tissues and returns them to the bloodstream.

Lymphatic Capillaries and Lymph

Lymphatic capillaries are the starting point of the lymphatic system, collecting excess interstitial fluid and transporting it as lymph. Lymph is a clear, colorless fluid similar to plasma but with less protein.

• Structure: Capillaries are closed at one end, held to surrounding tissue by protein filaments, and have loosely overlapped endothelial cells.

• Function: Allow bacteria and cells in; valve-like flaps open when interstitial fluid pressure is high and close when it is low.

Lacteals and Lipid Absorption

Lacteals are specialized lymphatic capillaries in the small intestine that absorb dietary fats and transport them to the circulatory system. The mixture of fat and lymph is called chyle, which appears milky.

• Villi: Finger-like projections in the small intestine contain lacteals for fat absorption.

• Chyle: Fat + lymph mixture transported by lacteals.

Lymphatic Vessels and Lymph Flow

Lymphatic vessels transport lymph from capillaries to the circulatory system. They are located superficial to blood vessels, contain more valves, and have thinner walls than veins. Lymph flows at low pressure and speed, aided by several mechanisms.

• Mechanisms: Skeletal muscle pump, thoracic pump, vessel pulsation, smooth muscle contraction, rapid blood flow in subclavian veins, and formation of new lymph.

• Valves: Prevent backward flow of lymph.

• Exercise: Increases lymphatic return.

Route of Lymph Flow

Lymph flows through a hierarchical route: lymphatic capillaries, collecting vessels (passing through lymph nodes), lymphatic trunks, and collecting ducts (right lymphatic duct and thoracic duct), finally draining into the subclavian veins.

• Right Lymphatic Duct: Receives lymph from the right arm, right side of head and thorax; empties into right subclavian vein.

• Thoracic Duct: Larger and longer; begins as cisterna chyli in the abdomen, receives lymph from below the diaphragm, left arm, left side of head, neck, and thorax; empties into left subclavian vein.

Lymphedema

Lymphedema is localized fluid retention and tissue swelling caused by a compromised or blocked lymphatic system. It can lead to tissue damage and increased risk of infections.

• Symptoms: Swelling, often in limbs.

• Causes: Blockage or damage to lymphatic vessels.

Lymphatic Organs

Red Bone Marrow

Red bone marrow is the site of hematopoiesis, the production of blood cells. As blood cells mature, they enter the bloodstream by passing through endothelial cells of blood vessels.

• Function: Produces red blood cells, white blood cells, and platelets.

Thymus Gland

The thymus gland is a bilobed organ located in the mediastinum. It has both lymphatic and endocrine functions and is essential for T-cell development. The thymus atrophies at puberty.

• T-cell Maturation: T lymphocytes mature in the thymus.

• Endocrine Function: Secretes hormones important for immune development.

Lymph Nodes

Lymph nodes are small, bean-shaped structures (<3 cm) that filter lymph and are sites of B and T cell activation. They notify immune cells of pathogens and are common sites for metastatic cancer.

• Structure: Cortex, medulla, capsule, afferent and efferent vessels.

• Lymphadenopathy: Collective term for lymph node diseases; includes lymphadenitis (swollen, painful node) and metastatic cancer (swollen, firm, painless).

Tonsils

Tonsils are lymphatic tissues that protect against inhaled and ingested pathogens. There are three types: palatine (back of oral cavity), lingual (root of tongue), and pharyngeal (adenoid, wall of pharynx).

• Function: Screening for microorganisms; pathogens enter tonsillar crypts and encounter lymphocytes.

• Swollen Tonsils: Often due to infection.

Spleen

The spleen filters blood, monitors for foreign antigens, and destroys aged red blood cells. It contains red pulp (sinuses filled with erythrocytes) and white pulp (lymphocytes, macrophages). The spleen is highly vascular and vulnerable to injury.

• Functions: Blood filtration, immune surveillance, destruction of old RBCs.

• Splenectomy: Surgical removal due to injury or disease.

The Immune System

Defenses Against Pathogens

The immune system provides two main types of defense: innate (nonspecific) and adaptive (specific). Innate defenses are broadly effective and do not require prior exposure, while adaptive defenses result from prior exposure and protect against particular pathogens.

• Innate Defenses: Surface barriers (skin, mucous membranes), internal defenses (phagocytes, NK cells, inflammation, antimicrobial proteins, fever).

• Adaptive Defenses: Humoral immunity (B cells), cellular immunity (T cells).

Innate Immunity: First Line of Defense

Innate immunity includes physical and chemical barriers such as skin and mucous membranes. Body secretions like lactic acid, lysozymes, sebum, gastric, and vaginal secretions help inhibit pathogen entry.

• Keratin: Provides a tough, protective barrier.

• Secretions: Contain antimicrobial substances.

Phagocytes

Phagocytes are cells that engulf and destroy pathogens. Neutrophils and macrophages are the primary phagocytes.

• Neutrophils: Wander through connective tissue, perform phagocytosis, and combat bacteria with respiratory burst.

• Macrophages: Derived from monocytes; specialized in various tissues (histiocytes, dendritic cells, alveolar, hepatic).

Natural Killer (NK) Cells

NK cells are responsible for immune surveillance, targeting cancer, viral, and bacterial cells. They attack diseased host cells and play a role in innate immunity.

Inflammation

Inflammation is a response to injury or infection, characterized by pain, redness, immobility, swelling, and heat. It is denoted by the suffix -itis (e.g., arthritis, dermatitis).

• Pain: Caused by inflammatory chemicals and pressure on nerves.

• Redness: Increased blood flow.

• Immobility: Loss of function.

• Swelling: Increased capillary permeability.

• Heat: Caused by hyperemia.

Complement System

The complement system consists of 30+ proteins in the blood that act in both nonspecific and specific immunity. These proteins are made by the liver and circulate in inactive form, becoming activated upon pathogen detection.

• Membrane Attack Complex: Complement proteins form a ring in the plasma membrane of target cells, causing cytolysis.

Fever (Pyrexia)

Fever is an abnormal elevation of body temperature, often triggered by interleukin-1 (pyrogen) released by neutrophils and macrophages after phagocytizing bacteria. Fever increases metabolism and tissue repair, inhibits pathogen reproduction, and is generally beneficial unless very high.

• Low-grade fever: May not require treatment.

• High fever: >105°F may cause delirium; 111°F-115°F can lead to coma or death.

Summary Table: Lymphatic Organs and Functions

Summary Table: Immune System Defenses